feat: allow numbers in plugin ID (#6656)

* fix example for id that didn't respect regex

example for id don't work with version 2.0.0-beta.15:
ValidationError: "id" with value "docs1" fails to match the required pattern: /^[a-zA-Z_-]+$/
Error: Process completed with exit code 1.

* properly fix

Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

packages/docusaurus-utils-validation/src/__tests__/validationSchemas.test.ts

@@ -69,6 +69,7 @@ describe('validation schemas', () => {
     testOK('docs');
     testOK('default');
     testOK('plugin-id_with-simple-special-chars');
+    testOK('doc1');
 
     testFail('/docs');
     testFail('docs/');