Fixes typo on src/pcontacs.cpp. Issue #4

Fixes issue #4: Typo on line 52 of src/pcontacs.cpp. acelleration -> acceleration.
Fixes issue #19
Merge branch 'master' of https://github.com/LucasCampos/cyclone-physics
Merge pull request #20 from LucasCampos/master

Fixes issue#4. Typo on pcontacs.cpp