pinctrl: remove enable/disable callbacks from documentation

Commit 03e9f0ca (pinctrl: clean up after enable refactoring) updated the
documentation to remove mention of disable(), and rename enable() to set_mux().
One in-text mention was forgotten. Fix this.

Fixes: 03e9f0ca ('pinctrl: clean up after enable refactoring')
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>