DOC: config: fix jwt_verify() example using var()

DOC: config: fix jwt_verify() example using var()

To prevent bogus matches, var() does not default to string type anymore
since 44c5ff6 ("MEDIUM: vars: make the var() sample fetch function really
return type ANY).

Thanks to the above fix, haproxy now returns an error if var() is used
within an ACL or IF condition and the matching type is not explicitly
set.

However, the documentation was not updated to reflect this change.

This partially fixes GH #2087 and must be backported up to 2.6.