change homepage in package.json to be a url (#154)

When using yarn, I see the url of this project listed as `http://.`,
this commit should fix that issue.

https://docs.npmjs.com/files/package.json#homepage

Author: terite

package.json

@@ -112,7 +112,7 @@
   "bugs": {
     "url": "https://github.com/bbc/react-transcript-editor/issues"
   },
-  "homepage": ".",
+  "homepage": "https://github.com/bbc/react-transcript-editor",
   "repository": {
     "type": "git",
     "url": "git+https://github.com/bbc/react-transcript-editor.git"