chisel: Use opt_libexec instead of libexec in caveats

In the caveats of chisel formula, now the following message is shown:
```
Add the following line to ~/.lldbinit to load chisel when Xcode launches:
  command script import /usr/local/Cellar/chisel/1.1.0/libexec/fblldb.py
```

However the path will change when upgrading.
So in this pull request I fix the caveats like the following:

```
Add the following line to ~/.lldbinit to load chisel when Xcode launches:
  command script import /usr/local/opt/chisel/libexec/fblldb.py
```