`string.format` missing from cel-go/ext/README.md

[amcrn]

Describe the bug

<string>.format is supported in the Strings extension (see https://github.com/google/cel-go/blob/master/ext/strings.go#L57-L102)

But it looks like it is accidentally omitted from the README: https://github.com/google/cel-go/blob/master/ext/README.md