Fix error message in optimizers piecewise_constant. (jax-ml#3061)

jax/experimental/optimizers.py

@@ -499,7 +499,7 @@ def piecewise_constant(boundaries, values):
   if not boundaries.ndim == values.ndim == 1:
     raise ValueError("boundaries and values must be sequences")
   if not boundaries.shape[0] == values.shape[0] - 1:
-    raise ValueError("boundaries length must be one longer than values length")
+    raise ValueError("boundaries length must be one shorter than values length")
 
   def schedule(i):
     return values[jnp.sum(i > boundaries)]