SDAM specification violation

[chowevtfy]

Noticed a spec violation while reading through the source:

https://github.com/comtihon/mongodb-erlang/blob/master/src/mongoc/mc_monitor.erl#L63 -

This line violates the following part of the spec ("This value MUST be 500 ms, and it MUST NOT be configurable"):

https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring.rst#minheartbeatfrequencyms

[comtihon]

Yes, got it. I'll fix.