Rename nnzJ to nJ in mjData.

This naming is more consistent.

PiperOrigin-RevId: 691858223
Change-Id: Ife3db822afa0b51b30d0792548b35b3742da388a

python/mujoco/structs.cc

@@ -747,7 +747,7 @@ void MjDataWrapper::Serialize(std::ostream& output) const {
   X(ncon);
   X(ne);
   X(nf);
-  X(nnzJ);
+  X(nJ);
   X(nefc);
   X(nisland);
   X(time);
@@ -824,7 +824,7 @@ MjDataWrapper MjDataWrapper::Deserialize(std::istream& input) {
   X(ncon);
   X(ne);
   X(nf);
-  X(nnzJ);
+  X(nJ);
   X(nefc);
   X(nisland);
   X(time);