Add missing backticks to Praefect dataloss examples

Adds missing backticks to delimit the code example. Splits the
command and output of an example to make copying easier using the
copy button on the code box.

doc/administration/gitaly/praefect.md

@@ -782,12 +782,15 @@ The Praefect `dataloss` sub-command helps identify lost writes by checking for u
 
 ```shell
 sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss [-virtual-storage <virtual-storage>]
+```
 
 If the virtual storage is not specified, every configured virtual storage is checked for data loss.
 
 ```shell
 sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss
+```
 
+```shell
 Virtual storage: default
   Current read-only primary: gitaly-2
   Previous write-enabled primary: gitaly-1