speedup bash completion loading suggestion in docs
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
```
Homebrew/brew,claui/brew,konqui/brew,MikeMcQuaid/brew,sjackman/homebrew,Homebrew/brew,konqui/brew,mahori/brew,nandub/brew,nandub/brew,claui/brew,vitorgalvao/brew,EricFromCanada/brew,mahori/brew,konqui/brew,MikeMcQuaid/brew,reitermarkus/brew,MikeMcQuaid/brew,JCount/brew,EricFromCanada/brew,vitorgalvao/brew,MikeMcQuaid/brew,mahori/brew,Homebrew/brew,mahori/brew,JCount/brew,reitermarkus/brew,nandub/brew,reitermarkus/brew,sjackman/homebrew,konqui/brew,JCount/brew,vitorgalvao/brew,Homebrew/brew,vitorgalvao/brew,JCount/brew,reitermarkus/brew,claui/brew,claui/brew,nandub/brew,EricFromCanada/brew,EricFromCanada/brew,sjackman/homebrew,sjackman/homebrew