Fix prefix of maint set/show per-command

Fix prefix of maint set/show per-command

I noticed this:

(gdb) apropos per-command
maintenance set per-command -- Per-command statistics settings
set per-command space -- Set whether to display per-command space usage
set per-command symtab -- Set whether to display per-command symtab statistics
set per-command time -- Set whether to display per-command execution time
maintenance show per-command -- Show per-command statistics settings
show per-command space -- Show whether to display per-command space usage
show per-command symtab -- Show whether to display per-command symtab statistics
show per-command time -- Show whether to display per-command execution time

The subcommands of "maintenance set per-command" are missing the
maintenance keyword.  This is because that command is registered with
the wrong prefix.  This patch fixes that.

gdb/ChangeLog:

	* maint.c (_initialize_maint_cmds): Fix prefix of maint set/show
	per-command.