i2c: mux: refer to i2c-mux.txt

Correct references to i2c-mux.txt which was previously mux.txt.

Also correct the spelling of relevant.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>