Fix example code for json::json_pointer initialization

[kfwu1999]

Description

In #4453, the compilation error of json::json_pointer p = "/nested/one"; is due to the implicit conversion, as the constructor for json_pointer is marked explicit (see following source code).

https://github.com/nlohmann/json/blob/b36f4c477c40356a0ae1204b567cca3c2a57d201/include/nlohmann/detail/json_pointer.hpp#L62-L64

Removing the explicit can allows to compile and pass the test, but i think it it might violate the design intentions.

Modification

Changed the example code in the documentation (https://json.nlohmann.me/features/json_pointer/#json-pointer-creation) from copy initialization to direct initialization.

Before

json::json_pointer p = "/nested/one";

After

json::json_pointer p("/nested/one");

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• [x] Changes are described in the pull request, or an existing issue is referenced.
• [x] The test suite compiles and runs without error.
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