Update README.md

[lancedouglas1]

Updating the git repo URL for cloning.

[sebrexmetaswitch]

Hi,

We'd actually prefer not to remove the SSH option for cloning the repo from our docs, since that's our preferred way to clone it. Instead, we've made a different change to outline both the options for doing so: https://github.com/Metaswitch/clearwater-docker/commit/e48f4f14ea5cb3bca4a1cb8af8d545d8533969f6