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

protoc-gen-go/generator/generator.go

@@ -1781,7 +1781,7 @@ 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