Correct the sysctl names in gpioths(4)

[otis]

Summary

Temperature is exposed via temperature' leaf, humidity via humidity`
leaf.

[pauamma_gundo.com]

English in change LGTM and appears consistent with sys/dev/gpio/gpioths.c to my untrained eye.

One thing I'd expect the likely audience (measurement/instrumentation devs or designers) to be interested in is the actual range and accuracy:

*   DHT11:  Temp   0c to 50c +-2.0c, Humidity 20% to  90% +-5%
*   DHT12:  Temp -20c to 60c +-0.5c, Humidity 20% to  95% +-5%
*   DHT21:  Temp -40c to 80c +-0.3c, Humidity  0% to 100% +-3%
*   DHT22:  Temp -40c to 80c +-0.3c, Humidity  0% to 100% +-2%
*   AM2301: Same as DHT21, but also supports i2c interface.
*   AM2302: Same as DHT22, but also supports i2c interface.

Not making them search for those in the kernel source would be good, I think.