[Docs] Fix short alias for 'unassigned.for' (#51059)

The short alias for `unassigned.for` is `uf`, not 'ua'.

docs/reference/cat/shards.asciidoc

@@ -239,7 +239,7 @@ Time (UTC)].
 `unassigned.details`, `ud`::
 Details about why the shard became unassigned.
 
-`unassigned.for`, `ua`::
+`unassigned.for`, `uf`::
 Time at which the shard was requested to be unassigned in
 https://en.wikipedia.org/wiki/List_of_UTC_time_offsets[Coordinated Universal
 Time (UTC)].
@@ -388,4 +388,4 @@ twitter 0 r STARTED    3014 31.1mb 192.168.56.30 bGG90GE
 twitter 0 r STARTED    3014 31.1mb 192.168.56.20 I8hydUG
 twitter 0 r UNASSIGNED ALLOCATION_FAILED
 ---------------------------------------------------------------------------
-// TESTRESPONSE[non_json]
+// TESTRESPONSE[non_json]