manual: replace an obsolete collation example with a valid one

In the Spanish language, the digraph "ll" has not been considered a
separate letter since 1994:
  https://www.rae.es/consultas/exclusion-de-ch-y-ll-del-abecedario

Since January 1998 (commit 49891c10),
glibc's locale data no longer specifies "ch" and "ll" as separate
collation elements.  So, it's better to not use "ll" in an example.

Also, the Czech "ch" is a better example as it collates in a more
surprising place.