Ruby: fix typo

Co-authored-by: Harry Maclean <hmac@github.com>

ruby/ql/lib/codeql/ruby/security/internal/CleartextSources.qll

@@ -209,7 +209,7 @@ module CleartextSources {
       )
     }
 
-    override string describe() { result = "an write to " + name }
+    override string describe() { result = "a write to " + name }
   }
 
   /** An assignment that may assign a password to a variable */