Update p001.cpp

[tigertv]

Hi!

Make it faster.

• [x] I checked CONTRIBUTING.md for my programming language of choice.

How the solution works

No loops. It uses sum of arithmetic progression.

Performance

https://tio.run/ gives that

Real time: 0.340 s
User time: 0.271 s
Sys. time: 0.061 s
CPU share: 97.96 %
Exit code: 0