Fix typo.

core/Archive.php

@@ -20,7 +20,7 @@ use Piwik\Period\Range;
  * The **Archive** class is used to query archive data.
  * 
  * You can use **Archive** instances to get archive data for one or more sites,
- * for one or more periods and on optional segment.
+ * for one or more periods and one optional segment.
  * 
  * If archive data is not found, this class will initiate the archiving process. [1](#footnote-1)
  *