Tweaks to CommonMark references

- Link to http://commonmark.org/ rather than http://spec.commonmark.org/ as it provides a more user-friendly landing point.
- "Common Markdown" → "CommonMark"
- Backticks around "description".

Author: wincent

spec/Section 4 -- Introspection.md

@@ -74,9 +74,10 @@ underscores.
 
 All types in the introspection system provide a `description` field of type
 `String` to allow type designers to publish documentation in addition to
-capabilities. A GraphQL server may return the `description` field using 
-[Common Markdown](http://spec.commonmark.org/) syntax. Therefore it is recommended
-that any tool that displays description use a Common Markdown compliant renderer.
+capabilities. A GraphQL server may return the `description` field using Markdown
+syntax (as specified by [CommonMark](http://commonmark.org/)). Therefore it is
+recommended that any tool that displays `description` use a CommonMark-compliant
+Markdown renderer.
 
 
 ### Deprecation