Fix Markdown syntax error in aufs-driver

Signed-off-by: dm4 <dm4@skymizer.com>

docs/userguide/storagedriver/aufs-driver.md

@@ -97,7 +97,7 @@ Storage Driver: aufs
  Dirperm1 Supported: false
 Execution Driver: native-0.2
 ...output truncated...
-````
+```
 
 The output above shows that the Docker daemon is running the AUFS storage driver on top of an existing ext4 backing filesystem.