nit: remove extra whitespace in NullValue comment

PiperOrigin-RevId: 512017807

src/google/protobuf/struct.proto

@@ -80,7 +80,7 @@ message Value {
 // `NullValue` is a singleton enumeration to represent the null value for the
 // `Value` type union.
 //
-//  The JSON representation for `NullValue` is JSON `null`.
+// The JSON representation for `NullValue` is JSON `null`.
 enum NullValue {
   // Null value.
   NULL_VALUE = 0;