runtime: Four-space indents for nested lists

Following opencontainers/runtime-spec@7795661 (runtime.md: Fix
sub-bullet indentation, 2016-06-08, opencontainers/runtime-spec#495).
From [1]:

  List items may consist of multiple paragraphs.  Each subsequent
  paragraph in a list item must be indented by either 4 spaces or one
  tab...

I expect that's intended to be read with "block element" instead
of "paragraph", in which case it applies to nested lists too.

And while GitHub supports two-space indents [2]:

  You can create nested lists by indenting lines by two spaces.

it seems that pandoc does not.

[1]: http://daringfireball.net/projects/markdown/syntax#list
[2]: https://help.github.com/articles/basic-writing-and-formatting-syntax/#lists

Signed-off-by: W. Trevor King <wking@tremily.us>

runtime.md

@@ -38,13 +38,13 @@ For example, POSIX systems define [`LANG` and related environment variables][pos
 Start a container from a bundle directory.
 
 * *Options*
-  * *`--id <ID>`* Set the container ID when creating or joining a container.
-    If not set, the runtime is free to pick any ID that is not already in use.
-  * *`--bundle <PATH>`* Override the path to the bundle directory (defaults to the current working directory).
+    * *`--id <ID>`* Set the container ID when creating or joining a container.
+      If not set, the runtime is free to pick any ID that is not already in use.
+    * *`--bundle <PATH>`* Override the path to the bundle directory (defaults to the current working directory).
 * *Standard streams:* The runtime MUST attach its standard streams directly to the application process without inspection.
 * *Environment variables*
-  * *`LISTEN_FDS`:* The number of file descriptors passed.
-    For example, `LISTEN_FDS=2` would mean that the runtime MUST pass file descriptors 3 and 4 to the application process (in addition to the [standard streams][standard-streams]) to support [socket activation][systemd-listen-fds].
+    * *`LISTEN_FDS`:* The number of file descriptors passed.
+      For example, `LISTEN_FDS=2` would mean that the runtime MUST pass file descriptors 3 and 4 to the application process (in addition to the [standard streams][standard-streams]) to support [socket activation][systemd-listen-fds].
 * *Exit code:* The runtime MUST exit with the application process's exit code.
 
 #### Example
@@ -63,11 +63,11 @@ $ echo $?
 Request the container state.
 
 * *Arguments*
-  * *`<ID>`* The container whose state is being requested.
+    * *`<ID>`* The container whose state is being requested.
 * *Standard streams:*
-  * *stdin:* The runtime MUST NOT attempt to read from its stdin.
-  * *stdout:* The runtime MUST print the state JSON to its stdout.
-  * *stderr:* The runtime MAY print diagnostic messages to stderr, and the format for those lines is not specified in this document.
+    * *stdin:* The runtime MUST NOT attempt to read from its stdin.
+    * *stdout:* The runtime MUST print the state JSON to its stdout.
+    * *stderr:* The runtime MAY print diagnostic messages to stderr, and the format for those lines is not specified in this document.
 * *Exit code:* Zero if the state was successfully written to stdout and non-zero on errors.
 
 #### Example