docs: write permission to Actions example to avoid 403 on push

[nanami-takemoto]

Summary

When setting up the GitHub Actions workflow from the README, the job may fail when trying to push the generated SVG files back to the repository.

Example error:

Permission to <repo> denied to github-actions[bot]

This happens when the repository uses read-only default workflow permissions, which is now the default setting for many repositories.
In that configuration, the GITHUB_TOKEN does not have permission to push commits.

Changes

The GitHub Actions example in the README now explicitly includes:

permissions:
  contents: write

This allows the workflow to push the generated files back to the repository.

Explicitly specifying this permission makes the example work out-of-the-box even for repositories using read-only default workflow permissions.
It also makes the required permission clearer for users who are new to GitHub Actions.

The same permission is also documented in the GitHub Actions version of this project:
https://github.com/stats-organization/github-readme-stats-action#readme

[vercel]

@nanami-takemoto is attempting to deploy a commit to the github readme stats Team on Vercel.

A member of the Team first needs to authorize it.