📝 [DOCS] Remove phrase "alternative...operating system" from PROS readme

[ion098]

I've seen people confused by the phrase "alternative open source operating system" in PROS' readme, which some people erroneously conflate with VEX's firmware (conveniently named VEXos), thus leading to confusion about PROS' competition legality with regards to <R15b>:

...Custom firmware modifications are not permitted.

Changing the readme to use an alternative term would help clear up this confusion, although I'm not sure what the best replacement term would be. The potential replacement phrases I can think of are:

• Alternative programming environment (I prefer this one personally, although it's not the most technically accurate)
• Alternative API
• Alternative hardware abstraction layer