vec: fix documentation of vec::each

5ec3aba (Improve documentation for each, 2012-12-27) incorrectly
documented vec::each by providing examples of old_iter::BaseIter usage
in the context of a vector.  Since d2e9912 (vec: remove BaseIter
implementation, 2013-06-21), those examples don't even work anymore.
Since vec::each() will be removed soon, provide examples using
`.iter().advance`.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>