Fixed documentation.

README.md

@@ -25,7 +25,7 @@ int main(void)
         res.set_content("Hello World!", "text/plain");
     });
 
-    svr_.get("/numbers/(\d+)", [&](const auto& req, auto& res) {
+    svr_.get(R"(/numbers/(\d+))", [&](const auto& req, auto& res) {
         string numbers = req.matches[1];
         res.set_content(numbers, "text/plain");
     });