Match CRuby exception message for Rational

Author: eregon

src/main/ruby/truffleruby/core/rational.rb

@@ -431,7 +431,7 @@ def marshal_load(ary)
   private :marshal_load
 
   def with_precision(method, n, **kwargs)
-    raise TypeError, 'not an Integer' unless Primitive.object_kind_of?(n, Integer)
+    raise TypeError, 'not an integer' unless Primitive.object_kind_of?(n, Integer)
 
     p = 10 ** n
     s = self * p