Merge branch 'docs/fix-codeblock-markdown' into 'master'

Fix indentation and markup for Markdown codeblock

See merge request gitlab-org/gitlab-ce!24321

doc/development/documentation/styleguide.md

@@ -272,7 +272,7 @@ Inside the document:
 - For regular code blocks, always use a highlighting class corresponding to the
   language for better readability. Examples:
 
-      ```md
+  ````md
   ```ruby
   Ruby code
   ```
@@ -284,7 +284,7 @@ Inside the document:
   ```md
   Markdown code
   ```
-      ```
+  ````
 
 - For a complete reference on code blocks, check the [Kramdown guide](https://about.gitlab.com/handbook/product/technical-writing/markdown-guide/#code-blocks).