Fix typo in FieldDescriptorProto.

PiperOrigin-RevId: 583411456

src/google/protobuf/descriptor.proto

@@ -294,12 +294,12 @@ message FieldDescriptorProto {
   // If true, this is a proto3 "optional". When a proto3 field is optional, it
   // tracks presence regardless of field type.
   //
-  // When proto3_optional is true, this field must be belong to a oneof to
-  // signal to old proto3 clients that presence is tracked for this field. This
-  // oneof is known as a "synthetic" oneof, and this field must be its sole
-  // member (each proto3 optional field gets its own synthetic oneof). Synthetic
-  // oneofs exist in the descriptor only, and do not generate any API. Synthetic
-  // oneofs must be ordered after all "real" oneofs.
+  // When proto3_optional is true, this field must belong to a oneof to signal
+  // to old proto3 clients that presence is tracked for this field. This oneof
+  // is known as a "synthetic" oneof, and this field must be its sole member
+  // (each proto3 optional field gets its own synthetic oneof). Synthetic oneofs
+  // exist in the descriptor only, and do not generate any API. Synthetic oneofs
+  // must be ordered after all "real" oneofs.
   //
   // For message fields, proto3_optional doesn't create any semantic change,
   // since non-repeated message fields always track presence. However it still