Fix gdb.base/memtag.exp failure

While running this test on an emulator, I noticed we're failing to match the
output message when "memory-tag check" is issued with no arguments.  That's
because I coded the message using "error" and missed a period at the end.  Other
similar messages are issued with error_no_arg.

This patch changes that call to use error_no_arg.

Tested on aarch64-linux Ubuntu 20.04/22.04.