Revert addition of Vin due to contradictory uses

It turns out that some people use Vin to mean "voltage input" (i.e. VBUS) and some people use it to mean "voltage internal" (i.e. VSYS), so just go back to the old diagram.

README.md

@@ -24,7 +24,7 @@ Keyboard only (incl. PC-XT) variant: https://github.com/No0ne/ps2pico
                   |                 |
 Pico GPIO11 ______| LV1         HV1 |______ PS/2 keyboard data
 Pico GPIO12 ______| LV2         HV2 |______ PS/2 keyboard clock
-Pico GPIO13 ______| LV          HV  |______ PS/2 5V + Pico VBUS/Vin
+Pico GPIO13 ______| LV          HV  |______ PS/2 5V + Pico VBUS
 Pico    GND ______| GND         GND |______ PS/2 GND
 Pico GPIO14 ______| LV3         HV3 |______ PS/2 mouse data
 Pico GPIO15 ______| LV4         HV4 |______ PS/2 mouse clock