Add syntax highlighting

Author: seaxwi

_linter/markdownlint/Rules.md

@@ -38,7 +38,7 @@ This rule is triggered when a valid [front matter](https://jekyllrb.com/docs/fro
 
 A front matter with the `title` variable only should look exactly like this:
 
-```
+```markdown
 ---
 title: "Some article title"
 ---
@@ -60,15 +60,15 @@ Fix it by adding the missing variable to the front matter.
 
 Note that the error can also be triggered by a mispelling, such as
 
-```
+```markdown
 ---
 tile: "Article title"
 ---
 ```
 
 instead of
 
-```
+```markdown
 ---
 title: "Article title"
 ---
@@ -88,15 +88,15 @@ Fix it by removing the specified variable from the front matter.
 
 Note that the error can also be triggered by a mispelling, such as
 
-```
+```markdown
 ---
 tile: "Article title"
 ---
 ```
 
 instead of
 
-```
+```markdown
 ---
 title: "Article title"
 ---