Updated nlohmann::basicjson::is_string (markdown)

nlohmann::basicjson::is_string.md

@@ -22,6 +22,41 @@ Constant.
 
 None. The function's noexcept-specification is `noexcept`.
 
+## Example
+
+```cpp
+#include <iostream>
+#include <json.hpp>
+
+int main()
+{
+    nlohmann::json j_null;
+    nlohmann::json j_boolean = true;
+    nlohmann::json j_number = 17;
+    nlohmann::json j_object = {{"one", 1}, {"two", 2}};
+    nlohmann::json j_array = {1, 2, 4, 8, 16};
+    nlohmann::json j_string = "Hello, world";
+
+    std::cout << std::boolalpha << j_null.is_null() << '\n';
+    std::cout << std::boolalpha << j_boolean.is_boolean() << '\n';
+    std::cout << std::boolalpha << j_number.is_number() << '\n';
+    std::cout << std::boolalpha << j_object.is_object() << '\n';
+    std::cout << std::boolalpha << j_array.is_array() << '\n';
+    std::cout << std::boolalpha << j_string.is_string() << '\n';
+}
+```
+
+### Output
+
+```
+true
+true
+true
+true
+true
+true
+```
+
 ## See also
 
 - [**is_null**](https://github.com/nlohmann/json/wiki/nlohmann::basicjson::is_null)<br>