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>

Documentation/git-gc.txt

@@ -59,8 +59,8 @@ If the number of packs exceeds the value of `gc.autoPackLimit`,
 then existing packs (except those marked with a `.keep` file
 or over `gc.bigPackThreshold` limit)
 are consolidated into a single pack.
-If the amount of memory is estimated not enough for `git repack` to
-run smoothly and `gc.bigPackThreshold` is not set, the largest
+If the amount of memory estimated for `git repack` to run smoothly is
+not available and `gc.bigPackThreshold` is not set, the largest
 pack will also be excluded (this is the equivalent of running `git gc`
 with `--keep-base-pack`).
 Setting `gc.autoPackLimit` to 0 disables automatic consolidation of