Use "markdown" for syntax highlighting on GitHub ("md" not recognized).

README.md

@@ -136,7 +136,7 @@ comments don't appear in the final markup):
 
 For example:
 
-```md
+```markdown
 <!-- markdownlint-disable MD037 -->
 deliberate space * in * emphasis
 <!-- markdownlint-enable MD037 -->
@@ -145,7 +145,7 @@ deliberate space * in * emphasis
 Changes take effect starting with the line a comment is on, so the following
 has no effect:
 
-```md
+```markdown
 space * in * emphasis <!-- markdownlint-disable --> <!-- markdownlint-enable -->
 ```
 

doc/CustomRules.md

@@ -63,7 +63,7 @@ A rule is implemented as an `Object` with four required properties:
 
 Linting the Markdown document:
 
-```md
+```markdown
 # Title
 
 Text *text* text.

doc/Rules.md

@@ -1258,13 +1258,13 @@ people who may not be able to see it.
 
 Alternate text is commonly specified inline as:
 
-```md
+```markdown
 ![Alternate text](image.jpg)
 ```
 
 Or with reference syntax as:
 
-```md
+```markdown
 ![Alternate text][ref]
 
 ...

test/long_lines.md

@@ -8,7 +8,7 @@ This line however, while very long, doesn't have whitespace after the 80th colum
 
     [This long line is comprised entirely of a link](http://example.com "But is inside a code block") {MD013}
 
-```md
+```markdown
 [This long line is comprised entirely of a link](http://example.com "But is inside a code block") {MD013}
 ```