[RFA v2 11/13] Improve "backtrace" help text

[Tom Tromey]

>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:

Pedro> On 08/14/2017 04:40 AM, Tom Tromey wrote:
>> +Usage: backtrace [QUALIFIERS]... [COUNT]\n\

Pedro> QUALIFIER, singular.   "..." already implies multiple instances.

I've fixed this locally.

Tom