doc: simplify format description of urlObject.auth

The "at sign" is properly the "commercial at" but let's just avoid the
whole naming issue by putting the symbol right in the sentence.

PR-URL: https://github.com/nodejs/node/pull/22324


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>