Fix README ci badge (#313)

Signed-off-by: Julian Orchard <hello@julianorchard.co.uk>
Co-authored-by: Anton Tolchanov <1687799+knyar@users.noreply.github.com>

README.md

@@ -2,7 +2,7 @@
 
 [![Go Reference](https://pkg.go.dev/badge/github.com/tailscale/terraform-provider-tailscale.svg)](https://pkg.go.dev/github.com/tailscale/terraform-provider-tailscale)
 [![Go Report Card](https://goreportcard.com/badge/github.com/tailscale/terraform-provider-tailscale)](https://goreportcard.com/report/github.com/tailscale/terraform-provider-tailscale)
-![Github Actions](https://github.com/tailscale/terraform-provider-tailscale/actions/workflows/ci.yml/badge.svg?branch=master)
+![Github Actions](https://github.com/tailscale/terraform-provider-tailscale/actions/workflows/ci.yml/badge.svg?branch=main)
 
 This repository contains the source code for the [Tailscale Terraform provider](https://registry.terraform.io/providers/tailscale/tailscale).
 This Terraform provider lets you interact with the [Tailscale](https://tailscale.com) API.