doc: fix the example of index activate command
doc: fix the example of index activate command

Executing the previous example command of index activate gave the error:
"fatal: gerrit: activate not found". This change adds the "index" to
this example command so that it can be executed correctly.

Bug: Issue 8775
Change-Id: I08fe58d6920810ff0950b1ad62440b930a456126

qtproject/qtqa-gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit