[gdb] Fix trailing whitespace in refcard.tex

[gdb] Fix trailing whitespace in refcard.tex

Fix:
...
gdb/doc/refcard.tex:314: trailing whitespace.
+b \opt{\it file\tt:}{\it line}&set breakpoint at {\it line} number \opt{in \it file}\par
gdb/doc/refcard.tex:344: trailing whitespace.
+enable \opt{{\it n}}&enable breakpoints
gdb/doc/refcard.tex:347: trailing whitespace.
+enable once \opt{{\it n}}&enable breakpoints \opt{or breakpoint {\it n}};
gdb/doc/refcard.tex:350: trailing whitespace.
+enable del \opt{{\it n}}&enable breakpoints \opt{or breakpoint {\it n}};
gdb/doc/refcard.tex:494: trailing whitespace.
+\qquad {\it command-list}&create new GDB command {\it cmd};
gdb/doc/refcard.tex:498: trailing whitespace.
+\qquad {\it help-text}&create online documentation
gdb/doc/refcard.tex:595: trailing whitespace.
+list {\it lines}&display source surrounding {\it lines},
gdb/doc/refcard.tex:629: trailing whitespace.
+statement.\cr
...

Verified by checking refcard.pdf.

Approved-By: Tom Tromey <tom@tromey.com>