Use cppreference by default

[jxu]

cppreference.com is a much better resource for accurate info, which is to be desired for a manpage. Compare

• https://cplusplus.com/reference/string/string/
• https://en.cppreference.com/w/cpp/string/basic_string

It's not even close. In particular, cplusplus doesn't have version info and info for recent versions past C++14.

Discussion: https://www.reddit.com/r/cpp/comments/d2agm7/httpwwwcpluspluscom_or_httpswwwcppreferencecom/

[jxu]

I was using an out-of-date version from ubuntu 20.04. Seems like it is the default now. https://github.com/aitjcize/cppman/commit/e0744f927c468eca6d628d84c065c8e40c00c7c3