minor grammar fix

Author: ashishpatelcs

docs/version-4-upgrade.md

@@ -5,7 +5,7 @@ AngularFire2 4.0 is a refactor of the AngularFire2 package which implements
 
 ### Removing `AngularFire` for Modularity
 
-Prior to 4.0, AngularFire2 did not take advantage of the Firebase SDK's modularity for tree shaking. The `AngularFire` service has now been removed and the library broken up into smaller `@NgModule`s:
+Prior to 4.0, AngularFire2 did not take advantage of the Firebase SDK's modularity for tree shaking. The `AngularFire` service has now been removed and the library is broken up into smaller `@NgModule`s:
 
 * `AngularFireModule`
 * `AngularFireDatabaseModule`