cmd/go: fix Go structs in -json documentation

"string" should really be "struct" in the structures describing the module.

Change-Id: I4e9cb12434bd33aa243622380c78e5e297d01d0b
Reviewed-on: https://go-review.googlesource.com/125638Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/alldocs.go

@@ -928,7 +928,7 @@
 // 		Indirect bool
 // 	}
 //
-// 	type Replace string {
+// 	type Replace struct {
 // 		Old Module
 // 		New Module
 // 	}

src/cmd/go/internal/modcmd/mod.go

@@ -99,7 +99,7 @@ Go types:
 		Indirect bool
 	}
 	
-	type Replace string {
+	type Replace struct {
 		Old Module
 		New Module
 	}