Fix Ender 2 display SCK, BTN_ENC pins (#15997)

Marlin/src/pins/stm32/pins_BTT_SKR_MINI_E3.h

      *      (MOSI) PB7  | · · | PB8  (LCD_RS)
      *    (LCD_A0) PB9  | · · | PA10 (BTN_EN2)
      *            RESET | · · | PA9  (BTN_EN1)
-     *   (BTN_ENC) PB6  | · · | PA15 (SCK)
+     *   (BTN_ENC) PB6  | · · | PB5  (SCK)
      *                   -----
      *                    EXP1
      */
     #define BTN_EN1        PA9
     #define BTN_EN2        PA10
+    #define BTN_ENC        PB6
+
     #define DOGLCD_CS      PB8
     #define DOGLCD_A0      PB9
-    #define DOGLCD_SCK     PA15
+    #define DOGLCD_SCK     PB5
     #define DOGLCD_MOSI    PB7
     #define FORCE_SOFT_SPI
     #define LCD_BACKLIGHT_PIN -1