Fix code block formatting in CONTRIBUTING.md

CONTRIBUTING.md

@@ -77,13 +77,19 @@ This section contains notes for Tildes code repository maintainers. If you are a
 2. Bump the `tildes/package.json` version on `master`. E.g. to `"1.101.0"`.
 3. Add a Git tag on the commit that bumped the `package.json` version:
 
-    $ git tag -a -m'Automatically insert meme GIF links every other post' v1.101.0
+```sh
+$ git tag -a -m'Automatically insert meme GIF links every other post' v1.101.0
+```
 
 4. Create new `staging-<version>` and `develop-<version>` branches for the **next** version:
 
-    $ git branch -c master staging-1.102
-    $ git branch -c master develop-1.102
+```sh
+$ git branch -c master staging-1.102
+$ git branch -c master develop-1.102
+```
 
 5. Push the new tag and branches to the main Tildes repo (assuming Git remote named `upstream`):
 
-    $ git push --follow-tags upstream master staging-1.102 develop-1.102
+```sh
+$ git push --follow-tags upstream master staging-1.102 develop-1.102
+```