[ruby/bigdecimal] [Doc] Fix the document of BigDecimal()

[ci-skip]

https://github.com/ruby/bigdecimal/commit/905d0345ec

ext/bigdecimal/bigdecimal.c

@@ -2830,7 +2830,7 @@ VpNewVarArg(int argc, VALUE *argv)
 }
 
 /* call-seq:
- *   BigDecimal(initial, digits, exception: true)
+ *   BigDecimal(initial, digits=0, exception: true)
  *
  * Create a new BigDecimal object.
  *