Add necessary quotes around private key

Without the quotes, this command fails on my system.

Author: jeaye

README.md

@@ -23,7 +23,7 @@ This buildpack takes a different approach and makes using the native Git way of
 
 3. Set `GIT_SSH_KEY` to the private SSH key that can access both your repo and its submodules:
 
-        heroku config:set GIT_SSH_KEY=$(cat ~/.ssh/id_rsa)
+        heroku config:set GIT_SSH_KEY="$(cat ~/.ssh/id_rsa)"
 
 4. Enjoy the ride :)