Replace FIXME in the MANIFEST with the correct description for the file.

We have 117 instances of the text FIXME in the core source code - it's not
actually good choice of text for a placeholder meaning "fix this before
pushing", as it's easy to miss the one that was added with this intent.

My mistake, spotted by ilmari.

MANIFEST

@@ -5890,7 +5890,7 @@ t/op/grep.t			See if grep() and map() work
 t/op/groups.t			See if $( works
 t/op/gv.t			See if typeglobs work
 t/op/hash.t			See if the complexity attackers are repelled
-t/op/hash-clear-placeholders.t	FIXME
+t/op/hash-clear-placeholders.t	Tests for corner cases in S_clear_placeholders
 t/op/hash-rt85026.t		See if hash iteration/deletion works
 t/op/hashassign.t		See if hash assignments work
 t/op/hashwarn.t			See if warnings for bad hash assignments work