README.md changed to gender-neutral pronouns (#961)

Changed 'him' to 'them' in README.md, which is more gender neutral.

Author: XenioxYT

fcm-notifications/README.md

@@ -63,4 +63,4 @@ This sample comes with a web-based UI for testing the function. To test it out:
  1. Install dependencies locally by running: `cd functions; npm install; cd -`
  1. Deploy your project using `firebase deploy`
  1. Open the app using `firebase open hosting:site`, this will open a browser.
- 1. Start following a user, this will send a notification to him.
+ 1. Start following a user, this will send a notification to them.