(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/tsconfig.json

@@ -1,5 +1,7 @@
 {
   "extends": "../../buildtools/tsconfig-base.json",
+  "files": [],
+  "include": [],
   "references": [
     { "path": "../../app/client" },
     { "path": "../../app/common" },