Update constants.py to fix caching typo

updated the config parameter of no_cashing to no_caching  

this may be a breaking change to some configs that have used this value

Author: alanbuxey

src/pyff/constants.py

@@ -290,7 +290,7 @@ class Config(object):
 
     caching_enabled = S("caching_enabled", default=True, typeconv=as_bool, info="enable caching?")
 
-    no_cashing = N('no_cashing', invert=caching_enabled, short='C', info="disable all caches")
+    no_caching = N('no_caching', invert=caching_enabled, short='C', info="disable all caches")
 
     daemonize = S("daemonize", default=True, cmdline=['pyffd'], info="run in background")