Docs: Fix wrong code usage in the Value access section of json_pointer.md

[Fallen-Breath]

The code usage demo in the Value access section of the JSON Pointer document does not work correctly

https://json.nlohmann.me/features/json_pointer/#value-access

json/docs/mkdocs/docs/features/json_pointer.md

Lines 62 to 78 in a259ecc

	```cpp
	// the JSON value from above
	auto j = json::parse(R"({
	"array": ["A", "B", "C"],
	"nested": {
	"one": 1,
	"two": 2,
	"three": [true, false]
	}
	})");
	// access values
	auto val = j["/"_json_pointer]; // {"array":["A","B","C"],...}
	auto val1 = j["/nested/one"_json_pointer]; // 1
	auto val2 = j.at[json::json_pointer("/nested/three/1")]; // false
	auto val3 = j.value[json::json_pointer("/nested/four", 0)]; // 0
	```

There are 3 mistakes in the code above:

1. (line 74) The json pointer "/" does not point to the root object. Instead, it points to the value with an empty string as the key at the root object. It should be replaced with json pointer ""

   json/tests/src/unit-json_pointer.cpp

   Lines 91 to 92 in a259ecc

   	// empty string access
   	CHECK(j[json::json_pointer("/")] == j[""]);

   json/tests/src/unit-json_pointer.cpp

   Lines 68 to 70 in a259ecc

   	// the whole document
   	CHECK(j[json::json_pointer()] == j);
   	CHECK(j[json::json_pointer("")] == j);

2. (line 76) Incorrect usage of nlohmann::basic_json::at. It's a function and does not support the [] operator

3. (line 77) Incorrect usage of nlohmann::basic_json::value. It's a function and does not support the [] operator. Additionally, the argument 0 as the default value for the value function is incorrectly placed

This PR resolves these 3 mistakes and makes the demo code work correctly

---

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[coveralls]

coverage: 100.0%. remained the same
when pulling 4d6cae7 on Fallen-Breath:patch-1
into a259ecc on nlohmann:develop.

[nlohmann]

Looks good to me.