fix: update broken POSIX recommendations link in README

[Yanhu007]

Fixes #463

The old link http://www.gnu.org/software/libc/manual/html_node/Argument-Syntax.html now redirects to the glibc manual homepage (https://sourceware.org/glibc/manual/) instead of the specific Argument Syntax page.

Updated to the current URL: https://sourceware.org/glibc/manual/latest/html_node/Argument-Syntax.html

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[tomasaschan]

Thanks! There's also #464 which does the same thing, but I'm happy to accept your PR instead just to have you get your first contribution in :)

Before we can do that, though, we need you to sign the CLA.