Doc: Fix custom ignore pattern example

[KSR-Yasuda]

The original pattern \%.*<10c is invalid (E383).

If you stick to the pattern of \%<Nc, \%<10c. is a valid pattern (matches chars 1-9), however, it won't work as you expect replacing each character one by one instead of the whole part of 1-9 chars.

So, ^.\{,10} is more simple, isn't it?