Capitalize "<TAB>" in require_record_target error

This changes require_record_target to say "<TAB>" rather than "<tab>".
I think capitalizing here is a bit more GNU-ish, based on Emacs usage
and one other case in gdb.

gdb/ChangeLog
2018-11-08  Tom Tromey  <tom@tromey.com>

	* record.c (require_record_target): Upper-case "<TAB>".