Fix comments in rest_client

This is to fix:
1. comments about RateLimitExceeded and OverLimit
2. comments about headers(including typo specifed)
which will affect
https://docs.openstack.org/tempest/latest/microversion_testing.html

Change-Id: Ib2138a14d61cee0aaf60507111aca24c2afff94d
1 file changed