i18n: replace "login" by "log in"

Spotted by Fito
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Prefsnologintext2/es

Change-Id: I87d2d4d7f6d5863f91e1072a6a1bb6f201371b6d
Merge "i18n: replace "login" by "log in""