Fix command syntax error in TripleO and Mistral docs
Fix command syntax error in TripleO and Mistral docs

One of the example commands in "TripleO and Mistral" docs had
incorrect syntax.

Change-Id: I2970e898fd92a623810384952ecaf19e3e0618b9
Closes-Bug: 1810294

openstack/tripleo-common,openstack/tripleo-common