Updated a warning string in upgrade.js

Summary:
I inserted an 'in' in the warning message that is displayed when the user's package.json file doesn't have a valid version number for react native.

Test plan
---------------
-run upgrade.js on a project with an invalid version number.
Closes https://github.com/facebook/react-native/pull/7314

Differential Revision: D3245839

fb-gh-sync-id: e38f3be0884d8c75f9610faced85d5c81c24a49e
fbshipit-source-id: e38f3be0884d8c75f9610faced85d5c81c24a49e

local-cli/upgrade/upgrade.js

@@ -60,7 +60,7 @@ module.exports = function upgrade(args, config) {
         } else {
           console.log(
             chalk.yellow(
-              'A valid version number for \'react-native\' is not specified your \'package.json\' file.'
+              'A valid version number for \'react-native\' is not specified in your \'package.json\' file.'
             )
           );
         }