doc: typo fix from e路相扶.

Author: agentzh

README.markdown

@@ -2491,7 +2491,7 @@ location /foo {
     set $a 32;
     set $b 56;
  
-    set_by_lua $res
+    set_by_lua $sum
         'return tonumber(ngx.arg[1]) + tonumber(ngx.arg[2])'
         $a $b;
  

doc/HttpLuaModule.wiki

@@ -1977,7 +1977,7 @@ Here is an example
         set $a 32;
         set $b 56;
  
-        set_by_lua $res
+        set_by_lua $sum
             'return tonumber(ngx.arg[1]) + tonumber(ngx.arg[2])'
             $a $b;