documentation: Bug fix in a simple example.

The second example under the manual heading Simple Examples does not work.
The call to next-minute-from errs because the second argument must be a
list. It can’t be the raw number 15.

Thanks to Colton Lewis.

* doc/mcron.texi: edited text.

doc/mcron.texi

@@ -214,7 +214,7 @@ hours? Edit the file to read
 (job
    '(next-minute-from
       (next-hour (range 0 24 2))
-      15)
+      '(15))
    "my-program")
 @end example