Spelling and consistency tweaks

[Gadgetoid]

As promised, the next part of #170 to sweep up spelling and word consistency fixes. I've listed all the possibly contentious changes below with brief rationalisation. Everything else should have been a cut-and-dry spelling fix.

Consistency Fixes

• Change "On board" and "onboard" to "on-board" ("onboard" means something else)
• Change "data sheet" to "datasheet" (we never use "data sheet" in the industry)
• Change "Makecode" to "MakeCode"
• Change "on chip" to "on-chip" (consistency with "on-board")
• Change "I2c" and "i2c" to "I2C"
• Change "pwm" to "PWM"
• Change "mbed" to "Mbed"

Spelling fixes

• Change "it's" to "its" where appropriate
• Plural "axis" to "axes"
• Miscellanous others