[PR] Fix typos in flow pragma reminder

Summary:
Minor, but useful.
Closes facebook#5965

Differential Revision: D7423650

Pulled By: mroch

fbshipit-source-id: 1ce2c78f5a9691891b723297ab326e7e05d2a4af

Author: kevinSuttle

website/en/docs/types/modules.md

@@ -21,7 +21,7 @@ export interface MyInterface { /* ... */ };
 import type Foo, {MyObject, MyInterface} from './exports';
 ```
 
-> ***Don't forget to mention `@flow` on top of file, otherwise flow won't report error.***.
+> ***Don't forget to mention `@flow` on top of file, otherwise flow won't report errors***.
 
 ### Importing and exporting values <a class="toc" id="toc-importing-and-exporting-values" href="#toc-importing-and-exporting-values"></a>