prime.rb: fix a grammo

* lib/prime.rb (prime?): [DOC] fix a grammo, a missing article.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46076 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/prime.rb

@@ -150,7 +150,7 @@ class Prime
   end
 
 
-  # Returns true if +value+ is prime number, else returns false.
+  # Returns true if +value+ is a prime number, else returns false.
   #
   # == Parameters
   #