design-autorepair.rst: use the same prefix everywhere

This commit changes the "ganeti:watcher:repair:pending" and
"ganeti:watcher:repair:result" tag prefixes to use
"ganeti:watcher:autorepair:{pending,result}", so that the same prefix is
used in all cases.

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>