message: docs: fix main README grammatical error
diff: `@superset-ui/legacy-*` packages are extracted from the classic [Apache Superset](https://github.com/apache/incubator-superset) and converted into plugins.
  These packages are extracted with minimal changes (almost as-is). They also depend on legacy API (`viz.py`) to function.
  -`@superset-ui/plugin-*` packages are newer and has higher quality in general.
  +`@superset-ui/plugin-*` packages are newer and higher quality in general.
  A key difference that they do not depend on `viz.py`
  (which contain visualization-specific python code) and interface with
  `/api/v1/query/` instead: a new generic endpoint instead meant to
  