doc/grepper.txt: suggestion about setting global grepper options is misleading

in doc/grepper.txt, I see

```
If you want to set global grepper options in your vimrc, use either of the
following two lines first:
>
let g:grepper = {} " initialize g:grepper with empty dictionary
runtime plugin/grepper.vim " initialize g:grepper with default values
<
```

This reads like I just need to pick one of the 2 lines, and so I go with the second one, and try the following in my vimrc:
```
runtime plugin/grepper.vim
let g:grepper.tools = ['git', 'rg']
```

But this fails and I get a `E121: Undefined variable: g:grepper` message. It doesnt look like the `let g:grepper = {}` line is not optional?

Open doc/grepper.txt and review the section describing global grepper options and the example vimrc configuration. Verify the documented initialization sequence in Vim, then revise the wording so the required setup is unambiguous and confirm the example no longer produces the reported undefined-variable error.

Written by the indexing model from the issue text.