Update prob.md

Author: jaspwr

competition/midi/prob.md

@@ -43,7 +43,7 @@ For example, in the following, only at most **1** note is being played at any on
 **Note that:**
 * If a note is turned on again while it's already held or turned off while it's already released, that message does nothing.
 * If a note **OFF** (`0x8c`) and **ON** (`0x9c`) message for different notes are sent at the same instant, those notes do not overlap. The note off message is processed first.
-* If there are the same number of **OFF** (`0x8c`) and **ON** (`0x9c`) messages at the same instant the totla number of notes playing is unchanged.
+* If there are the same number of **OFF** (`0x8c`) and **ON** (`0x9c`) messages at the same instant the total number of notes playing is unchanged.
 
 ## Output