[docs] 2.x-ism in builtins module doc

[Alexander Belopolsky]

In " ... most modules have the name __builtins__ (note the 's')", the
"(note the 's')" part is clearly a leftover from 2.x where builtins
was called __builtin__.  I suggest removing it.

See http://docs.python.org/dev/library/builtins.html