[gdb] Fix trailing whitespace in annotate.texinfo

Fix:
...
gdb/doc/annotate.texinfo:127: trailing whitespace.
+(gdb)
gdb/doc/annotate.texinfo:132: trailing whitespace.
+$
gdb/doc/annotate.texinfo:732: trailing whitespace.
+@code{step} or @code{continue},
gdb/doc/annotate.texinfo:738: trailing whitespace.
+is output.  When the program stops,
...

Verified by checking annotate.html.

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