fix: the default value of map_big_decimal_to_decimal128 is true, not false (#131)

Author: cperezabo

source/data-modeling/field-types.txt

@@ -313,9 +313,9 @@ You can use the ``BigDecimal`` type to store numbers with increased precision.
 {+odm+} stores ``BigDecimal`` values in two different ways, depending on the
 value you set for the ``Mongoid.map_big_decimal_to_decimal128`` configuration property:
 
-- If set to ``true``, {+odm+} stores ``BigDecimal`` values as BSON ``Decimal128``
+- If set to ``true`` (default), {+odm+} stores ``BigDecimal`` values as BSON ``Decimal128``
   values.
-- If set to ``false`` (default), {+odm+} stores ``BigDecimal`` values as strings.
+- If set to ``false``, {+odm+} stores ``BigDecimal`` values as strings.
 
 Consider the following limitations when setting the
 ``Mongoid.map_big_decimal_to_decimal128`` option to ``true``: