Remove call to deprecated BigDecimal.new (#119)

RUBY-1653 Replace BigDecimal.new with BigDecimal()

Author: jonekdahl

docs/tutorials/bson-v4.txt

@@ -227,7 +227,7 @@ decimal rounding with exact precision.
   BSON::Decimal128.new("1.28")
 
   # Instantiate with a BigDecimal
-  d = BigDecimal.new(1.28, 3)
+  d = BigDecimal(1.28, 3)
   BSON::Decimal128.new(d)
 
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