refactor: fix typo in neural_network.cpp

[eltociear]

Description of Change

fixed comment typo

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes: intialize -> initialize

[mrmagic2020]

Well done spotting the typo :)