Update gettxgps.md, units need fix

[rotorman]

I checked the getTxGPS() output with a NMEA GPS (u-Blox SAM-M8Q) and found that some units in the description here were wrong. Altitude output is in meters, step size is 1m.
Speed is output in 1cm/s and the GPS HDOP is output 100 times higher than the standard output is, where good signal is around 1 (see e.g. https://en.wikipedia.org/wiki/Dilution_of_precision_(navigation)#Meaning_of_DOP_Values[citation_needed] ).

[rotorman]

Please also check the return value statement:
"Return the internal GPS position or nil if no valid hardware found"

I tested and found that getTxGPS() returns nil only if internalgps build option is not baked in, but returns a table also when GPS hardware is NOT connected. If we look at OpenTX source code, then we can also see this:
https://github.com/opentx/opentx/blob/00cf3f87580ff2eb643c90cfc74550a8117b9654/radio/src/lua/api_general.cpp#L873-L892