cmd/go: add /v2 to go.mod require example

I added /v2 to go.mod require example

Fixes #28374

Change-Id: I74cca374838d106eb79acb9189a02fe9443962c0
Reviewed-on: https://go-review.googlesource.com/c/144917Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/alldocs.go

@@ -1605,7 +1605,7 @@
 //
 // 	module my/thing
 // 	require other/thing v1.0.2
-// 	require new/thing v2.3.4
+// 	require new/thing/v2 v2.3.4
 // 	exclude old/thing v1.2.3
 // 	replace bad/thing v1.4.5 => good/thing v1.4.5
 //

src/cmd/go/internal/modload/help.go

@@ -394,7 +394,7 @@ verb followed by arguments. For example:
 
 	module my/thing
 	require other/thing v1.0.2
-	require new/thing v2.3.4
+	require new/thing/v2 v2.3.4
 	exclude old/thing v1.2.3
 	replace bad/thing v1.4.5 => good/thing v1.4.5