Make plural entries correct

Changes three singular forms of messages already supporting plural.
Affected messages:
* echo-notification-alert
* echo-notification-message
* echo-overlay-title-overflow

First reported at:
* https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Echo-notification-alert/es
* https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Echo-notification-message/es

Change-Id: Ifcaade5e141c801c6e546d2098578c933d60434e
Merge "Make plural entries correct"