Fixed typo in README.md: corrected 'df - T' to 'df -T'. (#1644)

Author: tshefi

README.md

@@ -419,7 +419,7 @@ virsh console domain_name
 ```
 
 ### Determining your filesystem type
-If you're not sure what filesystem you have, try `df - T` and the second
+If you're not sure what filesystem you have, try `df -T` and the second
 column will include the type.
 
 ### Determining if your filesystem supports d_type