Fix typos.

PiperOrigin-RevId: 710678815
Change-Id: Iab6f603f8f4d782ac7f34140add39e1f37625167

src/engine/engine_forward.c

@@ -147,7 +147,7 @@ void mj_fwdPosition(const mjModel* m, mjData* d) {
     mj_collision(m, d);  // timed internally (POS_COLLISION)
   }
 
-  // have threadpool: inertia and collision on seperate threads
+  // have threadpool: inertia and collision on separate threads
   else {
     mjTask tasks[2];
     mjFwdPositionArgs forward_args;