docs: fix documentation for HTTP read/write timeouts

The documentation for these options has been incorrect ever since their
addition via commit 084dcb55. This broke upgrading a CoreOS stable
cluster to 410.0.0 because the user was using the example TOML config
which contains:

    http_write_timeout = 10

This was silently ignored in the old stable version which predates the
addition of these options. After upgrading etcd began failing with:

    Type mismatch for 'config.Config.http_write_timeout': Expected float
    but found 'int64’.

Original issue: https://github.com/coreos/update_engine/issues/45