Correct requestlog formatString commented default

[prenagha]

Jetty version(s)
10.0.9

Java version/vendor
17 Coretto

OS type/version
Mac 12.3.1

Description
Align the commented default jetty.requestlog.formatString mentioned in the requestlog.ini with the default used in the code (CustomRequestLog).
Currently the comment refers to a %B that isn't used by the code nor is mentioned as being valid by the documentation.
Also adjust the %a parameter option to indicate which form the code uses.

How to reproduce?
Add the requestlog module to Jetty, observe the commented defaults in requestlog.ini

[prenagha]

PR in a moment

[gregw]

@lachlan-roberts needs to be merged to 12

[lachlan-roberts]

done with 24c0e9d