Update grad of while_loop message.

The previous error message was misleading as of
jax-ml@ed8dbd2
(see jax-ml#2414 (comment)
for context).

jax/lax/lax_control_flow.py

@@ -480,7 +480,7 @@ def _while_partial_eval(trace: pe.JaxprTrace, *tracers: pe.Tracer, cond_nconsts:
 
 def _while_transpose_error(*_, **kwargs):
   raise ValueError("Reverse-mode differentiation does not work for lax.while_loop. "
-                   "Try using lax.scan, or lax.fori_loop with constant bounds.")
+                   "Try using lax.scan.")
 
 while_p = lax.Primitive('while')
 while_p.multiple_results = True