Update README.md

Author: avandalen

README.md

@@ -2,7 +2,7 @@
 The standard Arduino analogRead() takes about 112us for the AVR and 425us for the SAMD21, this is very slow. 
 The new function analogReadFast() takes just 21us. 
 
-##### Compatibility
+#### Compatibility
 - AVR boards: Arduino Uno, Nano, Mega, Leonardo, etc.
 - SAMD21 boards: Arduino Zero, SAM 15x15, etc.