document localPassC consistently with compiler; fix ANSI capitalization

[tersec]

Addresses the ANSI capitalization in nim-lang/website#279

Used the manual's example of localPassc:

{.localPassc: "-Wall -Werror".}

Compiled with:

nim c --stylecheck:error localpassc.nim

and got

localpassc.nim(1, 3) Error: 'localPassc' should be: 'localPassC'

Checked the capitalizations the compiler expected using versions:

Nim Compiler Version 1.6.4 [Linux: amd64]
Compiled at 2022-02-10
Copyright (c) 2006-2021 by Andreas Rumpf

active boot switches: -d:release

Nim Compiler Version 1.7.1 [Linux: amd64]
Compiled at 2022-04-13
Copyright (c) 2006-2022 by Andreas Rumpf

git hash: 98cebad7debddfb147ee22bc6f3d81221582c4d6
active boot switches: -d:release

{.localPassC: "-Wall -Werror".}

indeed works, stylecheck or otherwise.

So the compiler is expecting localPassC. Without stylecheck, it doesn't matter, but with stylecheck, the manual incorrectly specifies what the compiler will accept.