Update doublyll.cpp

Menu driven Program to create a doubly linked list with dynamic functionality
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 *  STATUS: INCOMPLETE
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 *  ISSUES:
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 *  1. Sort Function has not been added.
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 *  2. Swap Function is probably the worst piece of code ever written ,but, I cannot seem to be able to come up with a better way.
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 *  NOTE: List numbering starts from 0.