internal/filedesc: fix parsing of enforce_utf8 option

Remove spurious negation.

Change-Id: I484fa6fecda85943cdedd96a6c6f0f2349f6bfee
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/188338
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

internal/filedesc/desc_lazy.go

@@ -497,7 +497,7 @@ func (fd *Field) unmarshalOptions(b []byte) {
 				fd.L1.IsWeak = wire.DecodeBool(v)
 			case FieldOptions_EnforceUTF8:
 				fd.L1.HasEnforceUTF8 = true
-				fd.L1.EnforceUTF8 = !wire.DecodeBool(v)
+				fd.L1.EnforceUTF8 = wire.DecodeBool(v)
 			}
 		default:
 			m := wire.ConsumeFieldValue(num, typ, b)