set pretty-index-json in grinder

Author: kevmoo

test/compare_output_test.dart

@@ -54,6 +54,7 @@ void main() {
         '--example-path-prefix',
         'examples',
         '--no-include-source',
+        '--pretty-index-json',
         '--output',
         tempDir.path
       ];

testing/test_package_docs/index.json

@@ -208,7 +208,8 @@ updateTestPackageDocs() {
         '../test_package_docs',
         '--example-path-prefix',
         'examples',
-        '--auto-include-dependencies'
+        '--auto-include-dependencies',
+        '--pretty-index-json'
       ],
       runOptions: options);
 }