[sysid] Fix adjusted R² calculation

[calcmogul]

It hardcoded p to 2.

[calcmogul]

The C++ compiler accepts Greek letters but not superscripts.

[ncorrea210]

Which one? I tried it using the utf8 character U+00B2 found here: https://www.compart.com/en/unicode/U+00B2 and it compiled fine with g++. (I deleted the comment just before that because it seemed a little nit-picky and unnecessary, I was just wondering).

[calcmogul]

g++ accepts those as extensions to the standard-mandated character set, but MSVC does not. Clang will accept them with a flag.

[ncorrea210]

Ah, I'm not familiar with MSVC or clang so didn't know. Otherwise everything LGTM here.

[calcmogul]

Was going off of memory doing work for Sleipnir. For the compilers FRC supports, looks like only the FRC 2023 compiler doesn't like superscripts: https://godbolt.org/z/5x74qKKEf. Seems to be conformance-related, because later versions of x86 GCC also fail if -pedantic is passed.