Polish document content in README.md (open-telemetry#542)

* Fix wrong spelling in README.md

Fix wrong spelling in README.md

* Polish document content

README.md

@@ -60,16 +60,16 @@ for definition of maturity levels).
 Components marked `Stable` provide the following guarantees:
 
 - Field types, numbers and names will not change.
-- Service names and service package names will not change.
+- Service names and `service` package names will not change.
 - Service method names will not change. [from 1.0.0]
 - Service method parameter names will not change. [from 1.0.0]
 - Service method parameter types and return types will not change. [from 1.0.0]
 - Service method kind (unary vs streaming) will not change.
-- Names of messages and enums will not change. [from 1.0.0]
-- Numbers assigned to enum choices will not change.
-- Names of enum choices will not change. [from 1.0.0]
-- The location of messages and enums, i.e. whether they are declared at the top lexical
-  scope or nested inside another message will not change. [from 1.0.0]
+- Names of `message`s and `enum`s will not change. [from 1.0.0]
+- Numbers assigned to `enum` choices will not change.
+- Names of `enum` choices will not change. [from 1.0.0]
+- The location of `message`s and `enum`s, i.e. whether they are declared at the top lexical
+  scope or nested inside another `message` will not change. [from 1.0.0]
 - Package names and directory structure will not change. [from 1.0.0]
 - `optional` and `repeated` declarators of existing fields will not change. [from 1.0.0]
 - No existing symbol will be deleted.  [from 1.0.0]
@@ -79,12 +79,12 @@ with version number 1.0.0.
 
 The following additive changes are allowed:
 
-- Adding new fields to existing messages.
-- Adding new messages or enums.
-- Adding new choices to existing enums.
-- Adding new choices to existing oneof fields.
-- Adding new services.
-- Adding new methods to existing services.
+- Adding new fields to existing `message`s.
+- Adding new `message`s or `enum`s.
+- Adding new choices to existing `enum`s.
+- Adding new choices to existing `oneof` fields.
+- Adding new `service`s.
+- Adding new `method`s to existing `service`s.
 
 All the additive changes above must be accompanied by an explanation about how
 new and old senders and receivers that implement the version of the protocol