Update floyd_warshall.cpp

[JohhannasReyn]

Fix Segmentation fault due to the Graph being passed by value, and there not being a copy constructor. Adding the & for the Graph parameter changes the passing of the parameter to be passed by reference and circumvents the segmentation fault.

Description of Change

Checklist

• [x ] 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
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Notes: Added an '&' to pass the Graph parameter by reference to avoid the segmentation fault that was being produced prior to this change.

[realstealthninja]

Looks good. I dont think this prevents all the segfaults though

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