[Rust Server] Fix server-writing docs for rust-server

[richardwhiuk]

The generated README says

You can use the example server and client as a basis for your own code.
See below for [more detail on implementing a server](#writing-a-server).

but the Writing a server section was removed in #3552, and replaced by a briefer note under Using the generated library.

This fixes the link to point to a section that exists.

This fix was contributed by @kw217

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[kw217]

Thanks @richardwhiuk and @wing328 !