Documentation: i2c: add i2c-sysfs into index

Append i2c-sysfs to toctree in order to get rid of building warnings.

Fixes: 31df7195b1 ("Documentation: i2c: Add doc for I2C sysfs")
Signed-off-by: Hu Haowen <src.res@email.cn>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

Documentation/i2c/index.rst

@@ -17,6 +17,7 @@ Introduction
    busses/index
    i2c-topology
    muxes/i2c-mux-gpio
+   i2c-sysfs
 
 Writing device drivers
 ======================