bump link to API spec

Signed-off-by: David Harris <david.harris@weave.works>

Author: David Harris

README.md

@@ -9,7 +9,7 @@
 The source-controller is a Kubernetes operator, specialised in artifacts acquisition
 from external sources such as Git, Helm repositories and S3 buckets.
 The source-controller implements the
-[source.toolkit.fluxcd.io](https://github.com/fluxcd/source-controller/tree/master/docs/spec/v1beta1) API
+[source.toolkit.fluxcd.io](https://github.com/fluxcd/source-controller/tree/main/docs/spec/v1beta2) API
 and is a core component of the [GitOps toolkit](https://fluxcd.io/docs/components/).
 
 ![overview](docs/diagrams/source-controller-overview.png)