docs/xdc_access.md: update ProxyJump snippet, minor cleanup

* remove excess space
* use %r instead of <user> when possible
* jump host requires an individual entry when using an IdentityFile
Signed-off-by: Calvin Ardi <calvin@isi.edu>

docs/xdc_access.md

@@ -19,7 +19,7 @@ appropriate value.
 Host *-<project>
   Hostname %h
   User <user>
-  ProxyCommand ssh -p 2202 -i ~/.ssh/<id_rsa> -W %h:%p <user>@jumpc.mergetb.io
+  ProxyCommand ssh -p 2202 -i ~/.ssh/<id_rsa> -W %h:%p %r@jumpc.mergetb.io
   IdentityFile ~/.ssh/<id_rsa>
 ```
 
@@ -29,17 +29,22 @@ Then you can SSH into an XDC with
 ssh <xdc-name>-<experiment-name>-<project>
 ```
 
-such as 
+such as
 
 ```shell
 ssh xdc47-delta-quadrant
 ```
 
-For non-buggy SSH, you can simply use
+For non-buggy SSH (OpenSSH 7.3+), you can use the following instead
 
 ```shell
+Host jumpc.mergetb.io
+  User <user>
+  IdentityFile ~/.ssh/<id_rsa>
+
 Host *-<project>
   Hostname %h
   User <user>
-  ProxyJump <user>@jumpc.mergetb.io:2202
+  IdentityFile ~/.ssh/<id_rsa>
+  ProxyJump %r@jumpc.mergetb.io:2202
 ```