Fix: Warning for shadowed variables (#3188)

[kernie]

Simple variable renaming to avoid warning about shadowed variable

Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header file single_include/nlohmann/json.hpp. The whole process is described here.

[coveralls]

Coverage remained the same at 100.0% when pulling 256473c on kernie:develop into 7603046 on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!