Fix markup in decimal.rst.

Doc/library/decimal.rst

@@ -365,7 +365,7 @@ Decimal objects
       creating a Decimal instance from a string.
 
    .. versionchanged:: 2.7
-      The argument to the constructor is now permitted to be a :float:`instance`.
+      The argument to the constructor is now permitted to be a :class:`float` instance.
 
    Decimal floating point objects share many properties with the other built-in
    numeric types such as :class:`float` and :class:`int`.  All of the usual math