nixos/modules/services/x11/xserver.nix: fix minor typo

nixos/modules/services/x11/xserver.nix

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           [ '''
               Identifier      "Trackpoint Wheel Emulation"
               MatchProduct    "ThinkPad USB Keyboard with TrackPoint"
-              Option          "EmulateWheel"          "true
+              Option          "EmulateWheel"          "true"
               Option          "EmulateWheelButton"    "2"
               Option          "Emulate3Buttons"       "false"
             '''