Fix URL pointing to Gremlin docs (#970)
Fix URL pointing to Gremlin docs (#970)

The current URL http://gremlindocs.com/ points to a parked domain.
The actual docs are at https://tinkerpop.apache.org/gremlin.html

We can [skip ci] since this isn't modifying any code and won't affect CI.
cayleygraph/cayley,cayleygraph/cayley