docs/gitops-toolkit/source-watcher.md: Remove deprecated io/ioutil

[dholbach]

Follow-up from https://github.com/fluxcd/flux2/issues/1658

• [x] https://github.com/fluxcd/source-watcher/blob/bebe0f5f4265f9ae7fce90770322b571bb28f324/controllers/gitrepository_watcher.go
• [x] https://github.com/fluxcd/website/blob/ea2ccb00003611ec9062a5466e060d5114a3f4d7/content/en/docs/gitops-toolkit/source-watcher.md)

[snebel29]

@dholbach Those two above should address this issue.