Fix typo

Author: Eugene Leonovich

README.md

@@ -32,7 +32,7 @@ queue = require('queue')
 queue.start()
 
 local tube_name = 'foobar'
-if null == queue.tube[tube_name] then
+if nil == queue.tube[tube_name] then
     queue.create_tube(tube_name, 'fifottl')
 end
 ```

tests/Integration/instance.lua

@@ -14,7 +14,7 @@ queue = require('queue')
 queue.start()
 
 function create_tube(tube_name, tube_type, opts)
-    if null ~= queue.tube[tube_name] then
+    if nil ~= queue.tube[tube_name] then
         queue.tube[tube_name]:drop()
     end