Fix comment for init_channel_elem to refer to elem->channel_data, not elem->call_data

src/core/lib/channel/channel_stack.h

@@ -138,7 +138,7 @@ typedef struct {
      is_first, is_last designate this elements position in the stack, and are
      useful for asserting correct configuration by upper layer code.
      The filter does not need to do any chaining.
-     Implementations may assume that elem->call_data is all zeros. */
+     Implementations may assume that elem->channel_data is all zeros. */
   grpc_error* (*init_channel_elem)(grpc_channel_element* elem,
                                    grpc_channel_element_args* args);
   /* Destroy per channel data.