spec: s/char_lit/rune_lit/
spec: s/char_lit/rune_lit/

The spec talks explicitly about rune literals but the
respective production is still called char_lit for
historic reasons. Updated the two occurences.

Fixes issue 4602.

R=rsc, iant, r, ken
CC=golang-dev
https://codereview.appspot.com/7070048

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang