Fixed command bytes (#23)

Changed first bytes for "PreChargePeriod" and "VcomhDeselect" to be what they should be in the datasheet (0xD9 and 0xDB, respectively). 
This fix got the driver working properly for my hardware (raspberry pi zero w + WaveShare SH1106 HAT). 
Before this fix, the display would behave differently in based on what seemed to be background cosmic radiation; sometimes it would work perfectly, sometimes it would go dark after a few seconds of displaying something.

Author: bodyweightenergy

src/command.rs

@@ -74,10 +74,10 @@ impl Command {
                 ([0xD5, ((0xF & fosc) << 4) | (0xF & div), 0, 0, 0, 0, 0], 2)
             }
             Command::PreChargePeriod(phase1, phase2) => (
-                [0x22, ((0xF & phase2) << 4) | (0xF & phase1), 0, 0, 0, 0, 0],
+                [0xD9, ((0xF & phase2) << 4) | (0xF & phase1), 0, 0, 0, 0, 0],
                 2,
             ),
-            Command::VcomhDeselect(level) => ([0x35, (level as u8) << 4, 0, 0, 0, 0, 0], 2),
+            Command::VcomhDeselect(level) => ([0xDB, (level as u8) << 4, 0, 0, 0, 0, 0], 2),
             Command::Noop => ([0xE3, 0, 0, 0, 0, 0, 0], 1),
             Command::ChargePump(en) => ([0xAD, 0x8A | (en as u8), 0, 0, 0, 0, 0], 2),
         };