Fix escaped '%' check when building Printf.Format
Addresses
https://github.com/JuliaLang/julia/pull/32859#issuecomment-688826726.
The issue here was advancing the parsing position too far when an
escaped `'%'` was encountered after the first format specifier. If an
escaped `'%'` was then followed by another specifier, it was ignored due
to being classified as an "escaped" character.
