_content/doc/gc-guide: fix typos
_content/doc/gc-guide: fix typos

Change-Id: I819d0ec7231a29c813a8a2cf77244cbdd8f698a7
Reviewed-on: https://go-review.googlesource.com/c/website/+/418054
Auto-Submit: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website