doc: a code example misses a "return". thanks YuanSheng Wang for the patch in #572.

README.markdown

@@ -1430,7 +1430,7 @@ This directive can be freely mixed with all directives of the [ngx_http_rewrite_
 ```nginx
 
  set $foo 32;
- set_by_lua $bar 'tonumber(ngx.var.foo) + 1';
+ set_by_lua $bar 'return tonumber(ngx.var.foo) + 1';
  set $baz "bar: $bar";  # $baz == "bar: 33"
 ```
 

doc/HttpLuaModule.wiki

@@ -1167,7 +1167,7 @@ This directive can be freely mixed with all directives of the [[HttpRewriteModul
 
 <geshi lang="nginx">
     set $foo 32;
-    set_by_lua $bar 'tonumber(ngx.var.foo) + 1';
+    set_by_lua $bar 'return tonumber(ngx.var.foo) + 1';
     set $baz "bar: $bar";  # $baz == "bar: 33"
 </geshi>