docs(di): fix spelling (#3261)

Author: jsayol

public/docs/ts/latest/cookbook/dependency-injection.jade

@@ -786,7 +786,7 @@ a(id="parent-tree")
 
   `@SkipSelf` is essential for two reasons:
 
-  1. It tell the injector to start its search for a `Parent` dependency in a component *above* itself,
+  1. It tells the injector to start its search for a `Parent` dependency in a component *above* itself,
   which *is* what parent means.
 
   2. Angular throws a cyclic dependency error if we omit the `@SkipSelf` decorator.