_content/doc: remove 'For Go 1.15' subtitle from module reference
_content/doc: remove 'For Go 1.15' subtitle from module reference

At this point, we're planning to have a single version of this
document that describes functionality in the highest supported version
of Go and notes where that differs from lower versions.

Changes after this point describe functionality new in Go 1.16.

For golang/go#33637

Change-Id: I485287ad724deb367da07d149cc862b7aff74c30
Reviewed-on: https://go-review.googlesource.com/c/website/+/284812
Trust: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Run-TryBot: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

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