Fix selfcheck regression due to new maint command

Simon points out that the new maint command, intended to fix a
regression, also introduces a new regression in "maint selftest".

This patch fixes the error.  I did a full regression test on x86-64
Fedora 36.


diff --git a/gdb/dwarf2/cooked-index.c b/gdb/dwarf2/cooked-index.c
index d09faed..3a90769 100644
--- a/gdb/dwarf2/cooked-index.c
+++ b/gdb/dwarf2/cooked-index.c
@@ -659,8 +659,8 @@ _initialize_cooked_index ()
 
   add_cmd ("wait-for-index-cache", class_maintenance,
 	   maintenance_wait_for_index_cache, _("\
-Usage: maintenance wait-for-index-cache\n\
-Wait until all pending writes to the index cache have completed."),
+Wait until all pending writes to the index cache have completed.\n\
+Usage: maintenance wait-for-index-cache"),
 	   &maintenancelist);
 
   gdb::observers::gdb_exiting.attach (wait_for_index_cache, "cooked-index");