docs(attribute-directives): fix typo (#3193)

Change `Import` to `Input` which is used in the example
```
import { Directive, ElementRef, HostListener, Input } from '@angular/core';
```

public/docs/ts/latest/guide/attribute-directives.jade

@@ -172,7 +172,7 @@ a#respond-to-user
   and respond by setting or clearing the highlight color.
 
   Begin by adding `HostListener` to the list of imported symbols;
-  add the `Import` symbol as well because you'll need it soon.
+  add the `Input` symbol as well because you'll need it soon.
 +makeExample('attribute-directives/ts/src/app/highlight.directive.ts','imports')(format=".")
 
 :marked