Typo on GCC 14 porting_to page

[peter0x44 via Gcc]

I was reading the GCC 14 porting to page and I noticed:
 Alternatively, projects using using Autoconf could enable 
AC_USE_SYSTEM_EXTENSIONS.

"using using" should be "using".

I read over the rest and didn't notice anything else wrong.

Thanks,
Peter D.