[Rivet] Typo in Particle::abspid() method

[Christian Gumpert]

Dear RIVET developers,

I stumbled over some very basic typo in the Particle class at:

https://rivet.hepforge.org/trac/browser/include/Rivet/Particle.hh?rev=36978e3f6dc8615ab791fba5aba86396080a48d7#L76

I guess there is an abs() missing as the function just returns the PDG 
ID at the moment.

Cheers
Christian