Whitespace changes, making sure the example is correcly aligned

If desired, we could also replace spaces with [Box-drawing characters](https://en.wikipedia.org/wiki/Box-drawing_character), like this:

```
🗁 Documents folder
├─🗁 2019/                                   # By Year
│ └─🗁 My bank/
│   ├─🗎 Statement January.pdf
│   └─🗎 Statement February.pdf
└─🗁 Insurances/                             # Insurances
  ├─🗁 Healthcare 123/
  │ ├─🗎 2022-01-01 Statement January.pdf
  │ ├─🗎 2022-02-02 Letter.pdf
  │ └─🗎 2022-02-03 Letter.pdf
  └─🗁 Dental 456/
    └─🗎 2021-12-01 New Conditions.pdf
```

But rendering this with proper fixed-width characters can be troublesome on some systems.

docs/advanced_usage.md

@@ -400,7 +400,7 @@ structure as in the previous example above.
      Statement January.pdf
      Statement February.pdf
 
- Insurances/                           # Insurances
+Insurances/                             # Insurances
    Healthcare 123/
      2022-01-01 Statement January.pdf
      2022-02-02 Letter.pdf