Fix typo in model_api.md

|specifics| is not defined in this snippet.

TBR=treib@chromium.org

Bug: None
Change-Id: I5654c0230340e672f1f24347c1a0b0eb35431c89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315140Reviewed-by: Victor Vianna <victorvianna@google.com>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/master@{#791196}

docs/sync/model_api.md

@@ -230,7 +230,7 @@ void WriteLocalChange(std::string key, ModelData data) {
     change_processor()->Put(key, ModelToEntityData(data),
                             batch->GetMetadataChangeList());
   }
-  batch->WriteData(key, specifics->SerializeAsString());
+  batch->WriteData(key, data.specifics->SerializeAsString());
   store_->CommitWriteBatch(std::move(batch), base::Bind(...));
 }
 ```