doc: normalize Markdown code block info strings

Prior to this commit, Markdown fenced code blocks in Markdown
files had inconsistent info strings. This has been corrected to
standardize on the one shown in the CommonMark spec.

Refs: https://github.com/commonmark/commonmark-spec/blob/1103710025f340702edcb070be41f9982b6544d5/spec.txt#L131

PR-URL: nodejs#33542
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: DerekNonGeneric

doc/guides/releases.md

@@ -283,7 +283,7 @@ in the release branch (e.g. a release for Node.js v4 would be added to the
 
 The new entry should take the following form:
 
-```md
+```markdown
 <a id="x.y.x"></a>
 ## YYYY-MM-DD, Version x.y.z (Release Type), @releaser
 

tools/doc/README.md

@@ -4,7 +4,7 @@ Here's how the node docs work.
 
 Each type of heading has a description block.
 
-```md
+```markdown
 # module
 
 <!--introduced_in=v0.10.0-->