docs: Use assert on bind (on the TCP echo server example) (#813)

Author: nilFinx

docs/docs.lua

@@ -470,7 +470,7 @@ local doc = {
             local uv = require("luv") -- "luv" when stand-alone, "uv" in luvi apps
 
             local server = uv.new_tcp()
-            server:bind("127.0.0.1", 1337)
+            assert(server:bind("127.0.0.1", 1337))
             server:listen(128, function (err)
               assert(not err, err)
               local client = uv.new_tcp()