README.md: Fix 'wit' -> 'with' typo
README.md: Fix 'wit' -> 'with' typo

This snuck in with 9f35e49f (Updating README to sync with Makefile
deletions, 2015-05-19, #284).

swcarpentry/amy,vahtras/amy,pbanaszkiewicz/amy,shapiromatron/amy,swcarpentry/amy,pbanaszkiewicz/amy,vahtras/amy,wking/swc-amy,shapiromatron/amy,wking/swc-amy,swcarpentry/amy,pbanaszkiewicz/amy,vahtras/amy,wking/swc-amy,shapiromatron/amy,wking/swc-amy