Fix presubmit file path comparison

I recently added this presubmit to verify changes get replicated
between versions of the SDK and the paths of the current SDK
didn't get created correctly causing it to warn even when you
correctly replicated the changes.

Change-Id: If69e0375dfe097e0210c4249d77b49f0c3537a2c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/143640
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

PRESUBMIT.py

@@ -44,7 +44,7 @@ def _CheckSdkDdcRuntimeSync(input_api, output_api):
     runtime_lib = 'lib/_internal/js_dev_runtime/private/ddc_runtime/'
     for nnbd_file in files:
         if nnbd_file.startswith('sdk_nnbd/' + runtime_lib):
-            file = 'sdk/' + runtime_lib + nnbd_file[4:]
+            file = 'sdk/' + nnbd_file[9:]
             if not file in files:
                 unsynchronized_files.append(file)
     if unsynchronized_files: