Update en.json

Improve system message per request on translatewiki.net (edit 4220728). Admittedly I have no clue what the old message was meant to convey, so this is just an hopefully educated guess.
Merge pull request #48 from JeroenDeDauw/i18n-4220728

Update en.json
Merge branch 'master' of ssh://wmfgerrit/mediawiki/extensions/Maps