Fix grammar and clarify the description for the Default timezone setting under location settings page

When setting the Default timezone ("timezone") for a site under Site administration → Location → Location settings there are some problems with the text (in English at least).

Firstly the default is "Server's local time" but the descriptive text refers to the value "Server time". Secondly, it conflates time and timezones (i.e. surely the value should be "Server's local timezone"). Thirdly, it doesn't use the "operating system setting" but PHP's setting (as far as I can tell). Fourthly, there's a grammar error ("...the only the..").