extension: Fix typo

We actually append the build number (the `BUILD_ID` environment variable,
which originates from `github.run_number` in the CI), not the build date.

web/packages/extension/webpack.config.js

@@ -14,7 +14,7 @@ function transformManifest(content, env) {
     const buildDate = new Date().toISOString().substring(0, 10);
 
     // The extension marketplaces require the version to monotonically increase,
-    // so append the build date onto the end of the manifest version.
+    // so append the build number onto the end of the manifest version.
     manifest.version = process.env.BUILD_ID
         ? `${packageVersion}.${process.env.BUILD_ID}`
         : packageVersion;