gdb/doc: move @anchor off @item line

When building the GDB info manual I see this warning:

  gdb.texinfo:41447: warning: @anchor should not appear on @item line

And indeed line 41447 looks like this:

  @item @anchor{maint info breakpoints}maint info breakpoints

I propose moving the @anchor{...} part to the previous line which
silences the warning.

Approved-By: Eli Zaretskii <eliz@gnu.org>