Makefile: fix vtproto 'go install' command

Author: josharian

Makefile

@@ -38,7 +38,7 @@ mysqlsh:
 # $ protoc --version
 # libprotoc 3.19.1
 # $ go install google.golang.org/protobuf/cmd/protoc-gen-go@latest
-# $ go install github.com/planetscale/vtprotobuf/cmd/protoc-gen-go-vtproto
+# $ go install github.com/planetscale/vtprotobuf/cmd/protoc-gen-go-vtproto@latest
 proto: internal/plugin/codegen.pb.go internal/python/ast/ast.pb.go
 
 internal/plugin/codegen.pb.go: protos/plugin/codegen.proto