Kernel/LHCbMath: fix stupid typo

• fix a very stupid typo in PS2DPolSym::integrateY (Thanks to @tovsiann )

This MR for 2018-patches, run2-patches and master branches

• [2018-07-20 00:05] Validation started with lhcb-gauss-dev#1707
• [2018-07-20 00:07] Validation started with lhcb-g4r103#429
• [2018-07-20 00:08] Validation started with lhcb-2018-patches#279

merged

mentioned in commit 825cd501

changed the description

mentioned in commit 6981619f

mentioned in merge request !1412 (merged)

mentioned in commit 3014ff4f

mentioned in merge request !1413 (merged)