remove confusing phrase from DecimalField docs
The phrase "if it exists" was used in reference to the `decimal_places` argument to `DecimalField`, when in fact that field is required.
|@@ -476,7 +476,7 @@ A fixed-precision decimal number, represented in Python by a|
|.. attribute:: DecimalField.max_digits|
|The maximum number of digits allowed in the number. Note that this number|
|- must be greater than or equal to ``decimal_places``, if it exists.|
|+ must be greater than or equal to ``decimal_places``.|
|.. attribute:: DecimalField.decimal_places|