Update classtut.pod

Rakudo now appears to support at least enough autovivification to remove this line.

The program below does the right thing and the examples appear to work without the initialization.
perl6 -e 'my %h; push(%h<k>, "v"); %h.say'

That part of the code seems to date back to September 2012 or earlier.
Merge pull request #22 from ronaldxs/patch-1

Update classtut.pod