Fix type: char -> uint8_t (#336)

Author: Rotzbua

README.md

@@ -225,7 +225,7 @@ void drawProgressBar(uint16_t x, uint16_t y, uint16_t width, uint16_t height, ui
 void drawFastImage(int16_t x, int16_t y, int16_t width, int16_t height, const uint8_t *image);
 
 // Draw a XBM
-void drawXbm(int16_t x, int16_t y, int16_t width, int16_t height, const char* xbm);
+void drawXbm(int16_t x, int16_t y, int16_t width, int16_t height, const uint8_t *xbm);
 ```
 
 ## Text operations