correct Mixin module constant in code example

[abargnesi]

This is a small change to the processor class code example in ast/processor/mixin.rb. This small inaccuracy stood out in such a clear and concise library.

Kudos to you for your work on ast 🎉.

[coveralls]

Coverage remained the same at 100.0% when pulling 4f3c896 on abargnesi:correct-mixin-code-example into af07c3b on whitequark:master.