DOC: fix typo in 'commision' (#111)

Author: tankorsmash

backtesting/backtesting.py

@@ -958,7 +958,7 @@ def __init__(self,
 
         `cash` is the initial cash to start with.
 
-        `commission` is the commision ratio. E.g. if your broker's commission
+        `commission` is the commission ratio. E.g. if your broker's commission
         is 1% of trade value, set commission to `0.01`. Note, if you wish to
         account for bid-ask spread, you cam approximate doing so by increasing
         the commission, e.g. set it to `0.0002` for commission-less forex