Fix: Change BasicContext to data directive in decimal module documentation

Author: abkmystery

Doc/library/decimal.rst

@@ -1033,10 +1033,10 @@ New contexts can also be created using the :class:`Context` constructor
 described below. In addition, the module provides three pre-made contexts:
 
 
-.. class:: BasicContext
+.. data:: BasicContext
 
-   This is a standard context defined by the General Decimal Arithmetic
-   Specification.  Precision is set to nine.  Rounding is set to
+   This is a predefined context object defined by the General Decimal Arithmetic Specification.
+   Precision is set to nine.  Rounding is set to
    :const:`ROUND_HALF_UP`.  All flags are cleared.  All traps are enabled (treated
    as exceptions) except :const:`Inexact`, :const:`Rounded`, and
    :const:`Subnormal`.