Remove mention of UMD support for web workers (#962)

* Remove mention of UMD support for web workers

OMT only supports amd or esm as output, so the readme should not
mention umd as supported.

When executing microbundle with umd format and `--workers`, the web
worker will fail to be detected or bundled.

* Update README.md

Co-authored-by: Ryan Christian <33403762+rschristian@users.noreply.github.com>

README.md

@@ -261,7 +261,7 @@ This can be customized by passing the command line argument `--css-modules "[nam
 ### Building Module Workers
 
 Microbundle is able to detect and bundle Module Workers when generating bundles in the
-`es`, `umd` and `modern` formats. To use this feature, instantiate your Web Worker as follows:
+`esm` and `modern` formats. To use this feature, instantiate your Web Worker as follows:
 
 ```js
 worker = new Worker(new URL('./worker.js', import.meta.url), { type: 'module' });