protoc-gen-go/generator: fix misspelling (#732)

Change-Id: I908a183499bc539b722f7f32ae83f6662ea729aa
Cherry-Pick: github.com/golang/protobuf@7be3631955993a734965532f776bad7093f6fc9d
Original-Author: Cyrille Hemidy <cyrille.hemidy@gmail.com>
Reviewed-on: https://go-review.googlesource.com/c/151425
Reviewed-by: Damien Neil <dneil@google.com>

protoc-gen-go/generator/generator.go

@@ -1782,7 +1782,7 @@ func (g *Generator) defaultConstantName(goMessageType, protoFieldName string) st
 // oneofField - field containing list of subfields:
 // - oneofSubField - a field within the oneof
 
-// msgCtx contais the context for the generator functions.
+// msgCtx contains the context for the generator functions.
 type msgCtx struct {
 	goName  string      // Go struct name of the message, e.g. MessageName
 	message *Descriptor // The descriptor for the message