Fix amount of entries in the json object

[abbaswasim]

In the STL-like access section after the initial j.push_back() calls there is another j.emplace_back() call that makes the size == 4 not 3.

This is a minor documentation change. The following doesn't apply.

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• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
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Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
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• Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
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[coveralls]

Coverage remained the same at 100.0% when pulling ddbdb65 on abbaswasim:patch-1 into 176d8e2 on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks! Good catch!