Fix the location of the issue-state-transition.config in the docs

From the source code [1] it is obvious that this file should be created
under $GERRIT_SITE/etc/ and not directly under the $GERRIT_SITE.

[1] https://gerrit.googlesource.com/plugins/hooks-its/+/master/hooks-its/src/main/java/com/googlesource/gerrit/plugins/hooks/workflow/GerritHookFilterChangeState.java

Change-Id: I0eaefd4d6bdbcd1a91aa07d44eb371522f233003