doc: adjust example in qvm-pool man page

'xen' driver does not even exists.

doc/manpages/qvm-pool.rst

@@ -59,11 +59,11 @@ Options
 Examples
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-Create a pool backed by the default `xen` driver. 
+Create a pool backed by the `file` driver.
     
 ::
 
-    qvm-pool -o dir_path=/mnt/foo -a foo xen
+    qvm-pool -o dir_path=/mnt/foo -a foo file
 
 Authors
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