gc docs: clean grammar for "gc.bigPackThreshold"

gc docs: clean grammar for "gc.bigPackThreshold"

Clean up the grammar in the documentation for
"gc.bigPackThreshold". This documentation was added in 9806f5a7bf ("gc
--auto: exclude base pack if not enough mem to "repack -ad"",
2018-04-15).

Saying "the amount of memory estimated for" flows more smoothly than
the previous "the amount of memory is estimated not enough".

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>