docs: update 64-bit core.packedGitLimit default

We bumped the default in be4ca2905 (Increase
core.packedGitLimit, 2017-04-20) but never adjusted the
documentation to match.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -662,7 +662,8 @@ core.packedGitLimit::
 	bytes at once to complete an operation it will unmap existing
 	regions to reclaim virtual address space within the process.
 +
-Default is 256 MiB on 32 bit platforms and 8 GiB on 64 bit platforms.
+Default is 256 MiB on 32 bit platforms and 32 TiB (effectively
+unlimited) on 64 bit platforms.
 This should be reasonable for all users/operating systems, except on
 the largest projects.  You probably do not need to adjust this value.
 +