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
diff --git a/Documentation/cmd-index-activate.txt b/Documentation/cmd-index-activate.txt
index 85c89c7..4428d12 100644
--- a/Documentation/cmd-index-activate.txt
+++ b/Documentation/cmd-index-activate.txt
@@ -37,7 +37,7 @@
 Activate the latest change index:
 
 ----
-  $ ssh -p 29418 review.example.com gerrit activate changes
+  $ ssh -p 29418 review.example.com gerrit index activate changes
 ----
 
 GERRIT