speedup bash completion loading suggestion in docs

The current suggestion for users shells out to `brew --prefix` three separate times, which has a non-zero overhead (as per quick benchmark below, approx 20ms per invocation). By making a minor change, the first invocation can be stored in a local variable to reduce shell startup time.

```
$ hyperfine --warmup=3 'brew --prefix'       
Benchmark #1: brew --prefix
  Time (mean ± σ):      21.3 ms ±   1.5 ms    [User: 8.8 ms, System: 10.6 ms]
  Range (min … max):    20.3 ms …  37.0 ms    128 runs
```