docs: update example in serverless Plugin documentation

[pottekkat]

Current State

The current documentation of the serverless Plugin shows a simple example of a log function:

return function() ngx.log(ngx.ERR, \"serverless pre function\"); end 

Desired State

This could be updated with a non-trivial example that could show the capabilities of the Plugin better:

        "serverless-pre-function": {
            "phase": "rewrite",
            "functions" : [
                "return function(conf, ctx)
                    local core = require(\"apisix.core\")
                    if not ngx.var.arg_name then
                        local uri_args = core.request.get_uri_args(ctx)
                        uri_args.name = \"world\"
                        ngx.req.set_uri_args(uri_args)
                    end
                end"
            ]
        }

[Neilblaze]

@navendu-pottekkat please assign this to me.

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[kayx23]

Would it be sufficient to add two examples (register custom built-in variables and modify specific field in response body), same as the api7 docs, for this issue? If so, I can open a PR.