fixed a typo in an error message. thanks Matthieu Tourne for reporting it.

src/ngx_http_lua_variable.c

@@ -273,7 +273,7 @@ ngx_http_lua_var_set(lua_State *L)
 
     /* variable not found */
 
-    return luaL_error(L, "varaible \"%s\" not found for writing; "
+    return luaL_error(L, "variable \"%s\" not found for writing; "
                       "maybe it is a built-in variable that is not changeable "
                       "or you forgot to use \"set $%s '';\" "
                       "in the config file to define it first",