The ornamental design for a plug-in connector, as shown and described.
1. Plug-in connector
1.1: Perspective (front, right side)
1.2: Perspective (front, right side)
1.3: Perspective (front, left side)
1.4: Perspective (back, bottom, right side)
1.5: Perspective (bottom, rear)
1.6: Top
1.7: Perspective (front)
1.8: Back
The parts colored in white and delineated in broken lines are not part of the claimed design; right side of the design is a mirror image of the left side; the colors and color scheme appearing in the representations are not part of the claimed design.