df: set min width of "Used" column to 5

src/uu/df/src/columns.rs

@@ -197,6 +197,7 @@ impl Column {
         match column {
             // 14 = length of "Filesystem" plus 4 spaces
             Self::Source => 14,
+            Self::Used => 5,
             // the shortest headers have a length of 4 chars so we use that as the minimum width
             _ => 4,
         }