x/sync/singleflight: add example of using Group

[karelbilek]

The documentation for singleflightdoesn't show any example of using singleflight.Group.

Thus it's not immediately clear whether the recommended usage is struct or pointer.

(This is not a general discussion/question about the proper use of the library, it's a request for adding documentation for it.)

[heschi]

cc @bcmills

[gopherbot]

Change https://go.dev/cl/496535 mentions this issue: singleflight: example for using Group

[karelbilek]

That looks amazing!