doc: fix gitter URL
doc: fix gitter URL

renaming repo is a bit painful in gitter. we would lose all members: https://gitter.im/gitterHQ/topics/topic/580ef9ea472236f05c9e4788/what-happens-if-i-rename-something-on-github-org-repo
we might decide to do it anyway someday.

stratumn/go,stratumn/go