Update copyright year range in gdb.texinfo and refcard.tex

Update copyright year range in gdb.texinfo and refcard.tex

I missed those files which need to be updated manually when I did
the copyright year range update. The copyright.py script reminds
us of that fact with the following message at the end...

    REMINDER: Multiple copyright headers must be updated by hand:
       gdb/doc/gdb.texinfo
       gdb/doc/refcard.tex
       gdb/gdbarch.sh

... and somehow I missed this. This commit makes the change for
gdb.texinfo and refcard.tex. gdbarch.sh is being updated separately
by Andrew Burgess.

gdb/doc/ChangeLog:

	* gdb.texinfo: Update copyright year ranges.
	* refcard.tex: Likewise.