doc: make link relative in 1.12 cgo release notes

Change a link in the cgo section of the 1.12 release notes from

  https://golang.org/cmd/cgo ...

to

  /cmd/cgo/ ...

to uniform it with other links on the page, and to ensure correct
target when the page is displayed on tip.golang.org.

Change-Id: I7653a6ea15ce111a60929c7ae7e9fb0dc9515502
Reviewed-on: https://go-review.googlesource.com/c/156858Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.12.html

@@ -135,7 +135,7 @@ go vet -vettool=$(which shadow)
 	Go 1.12 will translate the C type <code>EGLDisplay</code> to the Go type <code>uintptr</code>.
 	This change is similar to how Go 1.10 and newer treats Darwin's CoreFoundation
 	and Java's JNI types. See the
-	<a href="https://golang.org/cmd/cgo/#hdr-Special_cases">cgo documentation</a>
+	<a href="/cmd/cgo/#hdr-Special_cases">cgo documentation</a>
 	for more information.
 </p>