Update Share.js. Fixed bad grammar

Summary:
Fix a typo in Share.js.
Closes https://github.com/facebook/react-native/pull/11033

Differential Revision: D4211724

Pulled By: ericvicenti

fbshipit-source-id: 6f0383568af474b21dfae5b208d21f6fd28ba7d4

Libraries/Share/Share.js

@@ -59,7 +59,7 @@ class Share {
   static share(content: Content, options: Options = {}): Promise<Object> {
     invariant(
       typeof content === 'object' && content !== null,
-      'Content must a valid object'
+      'Content to share must be a valid object'
     );
     invariant(
       typeof content.url === 'string' || typeof content.message === 'string',