_content/doc/gc-guide: explicitly set font size for axis labels
_content/doc/gc-guide: explicitly set font size for axis labels

Fixes golang/go#53850.

Change-Id: I0b9262391c992d03f316918775bff7b75baa009c
Reviewed-on: https://go-review.googlesource.com/c/website/+/417634
Auto-Submit: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Reviewed-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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