Update CONTRIBUTING.md to remove reference to package-lock.json (#47)

There is no package-lock.json in the repository

CONTRIBUTING.md

@@ -33,7 +33,7 @@ There's no formal test suite yet. For now the process is roughly:
   - [ ] An update to the [in-depth.md](https://github.com/v8/web-tooling-benchmark/blob/master/docs/in-depth.md) document. Add a new entry to that list for the new benchmark, which describes the tool and the concrete benchmark case.
   - [ ] Add a new file `src/foo-benchmark.js`, which includes the actual driver code for the benchmark (see the [`src/babylon-benchmark.js`](https://github.com/v8/web-tooling-benchmark/blob/master/src/babylon-benchmark.js) for example).
   - [ ] Add a new file `src/foo-benchmark.test.js`, which checks that the benchmark in `src/foo-benchmark.js` at least runs to completion.
-  - [ ] `npm install --save-exact` any necessary dependencies, and be sure to include both the `package.json` and the `package-lock.json` changes in your pull request.
+  - [ ] `npm install --save-exact` any necessary dependencies, and be sure to include the `package.json` changes in your pull request.
   - [ ] Put any assets used by the benchmark into the `third_party` folder and hook them up with the virtual file system in `src/vfs.js`.
 
 ## Sign the CLA