[DF] Correct doc format for ROOT::VecOps::Mean
[DF] Correct doc format for ROOT::VecOps::Mean

Add missing newline before each "Example code, ..."

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root