Fix deprecation text

I noticed one spot where deprecate_cmd is called with a second
argument that is not a command name.  This patch fixes the problem.

Regression tested on x86-64 Fedora 38.

gdb/symtab.c

diff --git a/gdb/symtab.c b/gdb/symtab.c
index 58648a8..0464b6d 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -7248,7 +7248,7 @@ the use of prologue scanners."),
 	       &maintenanceflushlist);
   c = add_alias_cmd ("flush-symbol-cache", maintenance_flush_symbol_cache_cmd,
 		     class_maintenance, 0, &maintenancelist);
-  deprecate_cmd (c, "maintenancelist flush symbol-cache");
+  deprecate_cmd (c, "maintenance flush symbol-cache");
 
   gdb::observers::new_objfile.attach (symtab_new_objfile_observer, "symtab");
   gdb::observers::all_objfiles_removed.attach (symtab_all_objfiles_removed,