DOC - Installation pages, curl command for retrieving keys

I noticed that on the getting_started/installing page in the Debian section the curl command for retrieving the cassandra GPG KEYS does not work (the URL gets redirected and no data is returned by curl).

It might be fixed by adding '-L' to the curl command or by changing www.apache.org to downloads.apache.org on that page (commit c93c2983cd3f did this previously but is no longer in the code).

I have created a pull request for this issue: https://github.com/apache/cassandra/pull/648