docs: fix verbose-bit-mask example

[jwong101]

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:

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`
}

[rustbot]

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[jwong101]

Also, this comment seems to be out of date:

rust-clippy/clippy_lints/src/operators/mod.rs

Lines 268 to 269 in d1b5fa2

	/// ### Known problems
	/// llvm generates better code for `x & 15 == 0` on x86

As LLVM seems to compile x.trailing_zeros() >= 4 and x & 15 == 0 to the same thing(when optimizations are enabled).

Godbolt for proof.

[Alexendoo]

Good catch, thanks!