dependency-injection.md: fix some typos (commons-app#1681)

Author: san1j

dependency-injection.md

@@ -1,6 +1,6 @@
 ## Overview
 
-At its core, dependency injection is just the principle of `"tell, dont ask"` put into practice; for instance, if a class needs to use the `MediaWikiApi`, it should be handed an instance of the classs rather than reaching out to get it.  This has the effect of decoupling code, making it easier to test and reuse.
+At its core, dependency injection is just the principle of `"tell, don't ask"` put into practice; for instance, if a class needs to use the `MediaWikiApi`, it should be handed an instance of the class rather than reaching out to get it.  This has the effect of decoupling code, making it easier to test and reuse.
 
 ## Dependency Injection in the Commons app