Kernel/LHCbMath: fix stupid typo (@ibelyaev)

Fix a very stupid typo in PS2DPolSym::integrateY (Thanks to @tovsiann )
Cherry picked from !1409 (merged) on 2018-patches

• [2018-07-21 00:10] Validation started with lhcb-lcg-dev3#592
• [2018-07-21 00:11] Validation started with lhcb-lcg-dev4#592
• [2018-07-21 00:11] Validation started with lhcb-gaudi-head#1937
• [2018-07-21 00:12] Validation started with lhcb-head#1925
• [2018-07-21 00:12] Validation started with lhcb-tdr-test#223
• [2018-07-21 03:32] Validation started with lhcb-lcg-dev4#592

merged

mentioned in commit aa5dc169