Add missing backtick on tone() reference page

Author: per1234

Language/Functions/Advanced IO/tone.adoc

@@ -59,7 +59,7 @@ Nothing
 
 [float]
 === Notes and Warnings
-If you want to play different pitches on multiple pins, you need to call `noTone()` on one pin before calling `tone() on the next pin.
+If you want to play different pitches on multiple pins, you need to call `noTone()` on one pin before calling `tone()` on the next pin.
 [%hardbreaks]
 
 --