[ticket/13086] String replaces 50 with 20

PHPBB3-13086
Merge pull request #2994 from Skouat/ticket/13086-olympus

[ticket/13086] Update ACP_MASS_EMAIL_EXPLAIN language key

* Skouat/ticket/13086-olympus:
  [ticket/13086] String replaces 50 with 20
Merge branch 'develop-olympus' into develop-ascraeus

* develop-olympus:
  [ticket/13086] String replaces 50 with 20

Conflicts:
	phpBB/language/en/acp/email.php