minor #936 Update apps.md (clarkeash)

This PR was merged into the 2.x branch.

Discussion
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Set the language for better syntax highlighting

Commits
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d4d1dad Update apps.md

Author: acrobat

doc/apps.md

@@ -36,7 +36,7 @@ $repositories = $client->api('apps')->listRepositories(456);
 
 ### List repositories for a given installation and user
 
-```
+```php
 $repositories = $client->api('current_user')->repositoriesByInstallation(456);
 ```