fix contributing relative links (#866)

CONTRIBUTING.md

@@ -6,7 +6,7 @@ Thank you for your interest in contributing to Narwhals! Any kind of improvement
 You can contribute to Narwhals in your local development environment, using python3, git and your editor of choice.
 You can also contribute to Narwhals using [Github Codespaces](https://docs.github.com/en/codespaces/overview) - a development environment that's hosted in the cloud.
 This way you can easily start to work from your browser without installing git and cloning the repo.
-Scroll down for instructions on how to use [Codespaces](https://github.com/narwhals-dev/narwhals/blob/c424c226fecba1a799e90b6ab59a76d7d9606896/CONTRIBUTING.md#working-with-codespaces).
+Scroll down for instructions on how to use [Codespaces](#working-with-codespaces).
 
 ## Working with local development environment
 
@@ -127,8 +127,8 @@ Go to the forked repository on your GitHub account - you'll find it on your acco
 Click on the green `Code` button and navigate to the `Codespaces` tab.
 Click on the green button `Create codespace on main` - it will open a browser version of VSCode,
 with the complete repository and git installed. 
-You can now proceed with the steps [4. Setting up your environment](https://github.com/narwhals-dev/narwhals/blob/c424c226fecba1a799e90b6ab59a76d7d9606896/CONTRIBUTING.md#4-setting-up-your-environment) up to [8. Pull request](https://github.com/narwhals-dev/narwhals/blob/4052abd24bf6db5d71c5562722f7283eba3420c7/CONTRIBUTING.md#8-pull-requests)
-listed above in [Working with local development environment](https://github.com/narwhals-dev/narwhals/blob/4052abd24bf6db5d71c5562722f7283eba3420c7/CONTRIBUTING.md#8-pull-requests).
+You can now proceed with the steps [4. Setting up your environment](#4-setting-up-your-environment) up to [8. Pull request](#8-pull-requests)
+listed above in [Working with local development environment](#working-with-local-development-environment).
 
 
 ## How it works