(core) fix inconsistent name of directory for DocUsageBanner.ts

Summary:
Usage banner is expected at app/client/components, not
app/client/component. Unclear why core smoke test did not catch
this.

Test Plan: manual

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3458

stubs/app/client/components/DocUsageBanner.ts

@@ -0,0 +1,10 @@
+import {DocPageModel} from 'app/client/models/DocPageModel';
+import {Disposable} from 'grainjs';
+
+export class DocUsageBanner extends Disposable {
+  constructor(_docPageModel: DocPageModel) { super(); }
+
+  public buildDom() {
+    return null;
+  }
+}