docs(@schematics/angular): reference vitest-base config in karma-to-vitest migration warning

Author: itsdevdom

packages/schematics/angular/migrations/migrate-karma-to-vitest/karma-processor.ts

@@ -158,7 +158,8 @@ export async function processKarmaConfig(
       context.logger.warn(
         `Project "${projectName}" uses a custom Karma configuration file "${karmaConfig}". ` +
           `Tests have been migrated to use Vitest, but you may need to manually migrate custom settings ` +
-          `from this Karma config to a Vitest config (e.g. vitest.config.ts).`,
+          `from this Karma config to a Vitest config (e.g. "vitest-base.config.ts") ` +
+          `and set the "runnerConfig" option to true.`,
       );
       manualMigrationFiles.push(karmaConfig);
     }