suppress warnings for `brew doctor`

I got annoying warning at brew doctor, it looks are following:

```
% brew doctor
Warning: Some directories in your path end in a slash.
Directories in your path should not end in a slash. This can break other
doctor checks. The following directories should be edited:
    /Users/shimada.yuji/.plenv/versions/5.16.2/bin/
```

I fixed it.