cmd/vet: fix documentation for -structtags

Was missing a title in the documentation, so it formatted wrong.

Fixes #17124

Change-Id: Ie8a9c36fbc54eed7d8a761f89a088e582b8c062d
Reviewed-on: https://go-review.googlesource.com/29340Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/vet/doc.go

@@ -157,6 +157,8 @@ Flag: -shift
 
 Shifts equal to or longer than the variable's length.
 
+Struct tags
+
 Flag: -structtags
 
 Struct tags that do not follow the format understood by reflect.StructTag.Get.