[Avocado] Allow specs with `readme.md` in sub-folders under `resource-manager`

PR https://github.com/Azure/azure-rest-api-specs-pr/pull/21133 adds a `readme.md` like this:

```
specification/foo/resource-manager/Private.Foo/bar/readme.md
```

It does **not** have a `readme.md` directly under `resource-manager`. This causes check `Avocado` to fail:

```
{
"level":"Error",
"code":"MISSING_README",
"message":"Can not find readme.md in the folder. If no readme.md file, it will block SDK generation.",
"path":"/mnt/vss/_work/1/sha/specification/foo/resource-manager",
"folderUrl":"/mnt/vss/_work/1/sha/specification/foo/resource-manager"
}
avocadoForDir: /mnt/vss/_work/1/sha/specification/foo/resource-manager/Private.Foo/bar
errors: 1
```

https://github.com/Azure/azure-rest-api-specs-pr/pull/21133/checks?check_run_id=36616007878

Should this cause `Avocado` to fail, or is a bug in the check?

Other checks like `LintDiff` appear to find the `readme.md` correctly:

```
RETURN definition getChangingConfigureFiles
all affected readme are:
[
"specification/foo/resource-manager/Private.Foo/bar/readme.md"
]
```

https://dev.azure.com/azure-sdk/internal/_build/results?buildId=4529937&view=logs&j=0574a2a6-2d0a-5ec6-40e4-4c6e2f70bea2&t=80c3e782-49f0-5d1c-70dd-cbee57bdd0c7&l=61

CC: @weshaggard, @JeffreyRichter