docs: remove old caveat in README.md

[mindovermiles262]

Description

Cleans up readme for installing with 'latest' feature tag

Motivation and Context

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Usage examples

$ asdf install gcloud latest
$ asdf global gcloud latest
$ gcloud --version
Google Cloud SDK 367.0.0
bq 2.0.72
core 2021.12.10
gsutil 5.5

How Has This Been Tested?

Installed fresh plugin, then ran install latest and global latest.

Checklist:

• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.