doc: update DataDirectoryGroupReadable 1 in tor.md

Move DataDirectoryGroupReadable 1 up a few lines to more clearly
communicate that it is required for the filesystem group to read the
DataDirectory.

Per the Tor documentation
https://2019.www.torproject.org/docs/tor-manual.html.en#DataDirectoryGroupReadable
"If this option is set to 0, don’t allow the filesystem group to read
the DataDirectory. If the option is set to 1, make the DataDirectory
readable by the default GID. (Default: 0)"

Author: jessebarton

doc/tor.md

@@ -92,19 +92,13 @@ out by default (if not, add them):
 ControlPort 9051
 CookieAuthentication 1
 CookieAuthFileGroupReadable 1
+DataDirectoryGroupReadable 1
 ```
 
 Add or uncomment those, save, and restart Tor (usually `systemctl restart tor`
 or `sudo systemctl restart tor` on most systemd-based systems, including recent
 Debian and Ubuntu, or just restart the computer).
 
-On some systems (such as Arch Linux), you may also need to add the following
-line:
-
-```
-DataDirectoryGroupReadable 1
-```
-
 ### Authentication
 
 Connecting to Tor's control socket API requires one of two authentication