Update parse_exceptions.md - correct json::exception::parse_error

[frasermarlow]

Referring to https://github.com/nlohmann/json/blob/develop/doc/examples/parse_error.cpp and https://json.nlohmann.me/home/exceptions/
(and also based on testing), the catch command should reference catch (json::parse_error& ex) and not catch (json::exception::parse_error& ex)

[Describe your pull request here. Please read the text below the line, and make sure you follow the checklist.]

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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.
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• 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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[coveralls]

Coverage remained the same at 100.0% when pulling 90d51b9 on frasermarlow:patch-1 into 176d8e2 on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!