mux: fix grammar, missing "is".

This makes it easier to parse.
Signed-off-by: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/75067db8-0264-c49c-daa8-d7a877932711@axentia.seSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/mux/core.c

@@ -416,7 +416,7 @@ EXPORT_SYMBOL_GPL(mux_state_select_delay);
  * @delay_us: The time to delay (in microseconds) if the mux state is changed.
  *
  * On successfully selecting the mux-control state, it will be locked until
- * mux_control_deselect() called.
+ * mux_control_deselect() is called.
  *
  * Therefore, make sure to call mux_control_deselect() when the operation is
  * complete and the mux-control is free for others to use, but do not call