chore(mkdir): fix misleading comment about backspace character

Correct inaccurate comment in test_mkdir_control_characters.
The test was actually checking space characters in directory names,
not backspace characters as the comment incorrectly stated.

This addresses reviewer feedback about the confusing comment.

Author: naoNao89

tests/by-util/test_mkdir.rs

@@ -580,7 +580,7 @@ fn test_mkdir_control_characters() {
         assert!(at.dir_exists("test\tname"));
     }
 
-    // Test backspace (space in name - should work on all systems)
+    // Test space character in directory name (should work on all systems)
     scene.ucmd().arg("-p").arg("test name").succeeds();
     assert!(at.dir_exists("test name"));