Fixed missing ground pad reference

EESCHEMA/Custom_Parts.lib

@@ -1150,6 +1150,7 @@ X PC15/OSC 3 -700 550 200 R 50 50 1 1 I
 X PB7 30 700 550 200 L 50 50 1 1 I
 X PH3/BOOT0 31 700 650 200 L 50 50 1 1 I
 X VSS 32 700 750 200 L 50 50 1 1 I
+X VSS 33 700 750 200 L 50 50 1 1 I N
 X NRST 4 -700 450 200 R 50 50 1 1 I
 X VDDA 5 -700 350 200 R 50 50 1 1 I
 X PA0 6 -700 250 200 R 50 50 1 1 I