enumerator.c: [DOC] small fix

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65042 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

enumerator.c

@@ -2414,7 +2414,7 @@ stop_result(VALUE self)
  *
  * Enumerator::ArithmeticSequence is a subclass of Enumerator,
  * that is a representation of sequences of numbers with common difference.
- * The instances of this class can be generated by Range#step and Numeric#step
+ * Instances of this class can be generated by the Range#step and Numeric#step
  * methods.
  */