docs: fix verbose-bit-mask example

changelog: none

`x & 15 == 0` is not equivalent to `x.trailing_zeros() > 4`, as `x = 0b10000` is true for
the former and false for the latter.

In fact, clippy itself suggests the following:

```rust
pub fn src(x: i32) -> bool {
    x & 15 == 0 // ~error: bit mask could be simplified with a call to `trailing_zeros`
    ^^^^^^^^^^^ help: try: `x.trailing_zeros() >= 4`
}
```

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