Fix typo in BusIn.md

Make change from #488.

Author: Amanda Butler

docs/reference/api/drivers/BusIn.md

@@ -8,7 +8,7 @@ You can use any of the numbered Arm Mbed pins as a DigitalIn in the BusIn.
 
 **Tips:**
 * You can have up to 16 pins in a Bus.
-* The order of pins in the constructor is the reverse order of the pins in the byte order. If you have `BusOut(a,b,c,d,e,f,g,h)` then the order of bits in the byte would be `hgfedcba` with `a` being bit 0, `b` being bit 1, `c` being bit 2 and so on.
+* The order of pins in the constructor is the reverse order of the pins in the byte order. If you have `BusIn(a,b,c,d,e,f,g,h)` then the order of bits in the byte would be `hgfedcba` with `a` being bit 0, `b` being bit 1, `c` being bit 2 and so on.
 
 ### BusIn class reference