Syntax: Use plain-out rule for scope matching

The comment seems to have been copy-pasted from the include list, as I
can't imagine how a scope name in `expect_scope` would ever be part of
a context-list.

Fixes #307.

Package/Sublime Text Syntax Definition/Sublime Text Syntax Definition.sublime-syntax

@@ -302,8 +302,7 @@ contexts:
     - match: (?={{ns_plain_first_plain_out}})
       set:
         - meta_scope: meta.scope.sublime-syntax string.unquoted.plain.out.yaml
-        - match: '{{_flow_scalar_end_plain_in}}'
-          # use plain-in for when inside of an 'context-list', although not always accurate
+        - match: '{{_flow_scalar_end_plain_out}}'
           set:
             - match: \n?
               scope: meta.scope.sublime-syntax

Package/Sublime Text Syntax Definition/syntax_test_sublime-syntax.yaml

@@ -271,6 +271,9 @@ contexts:
 #       ^^^^^^^^^^^^^^^^^^^ meta.scope.sublime-syntax string.unquoted.block.yaml
 #           ^ punctuation.separator.scope-segments.sublime-syntax
 
+      scope: meta.interface.xxx, entity.name.xxx, etc.xxx
+#                              ^ string.unquoted.plain.out.yaml
+
     - embed: scope:source.test
 #     ^^^^^ string.unquoted.plain.out.yaml keyword.operator.include.sublime-syntax
 #            ^^^^^ support.type.include.sublime-syntax