Clarify sentence in SPI.md

Clarify sentence to resolve request in comments.

Author: Amanda Butler

docs/api/drivers/SPI.md

@@ -29,6 +29,6 @@ The SPI master generates a clock to synchronously drive a serial bit stream slav
 
 ### SPI hello, world
 
-The following example uses the WHOAMI register. The WHOAMI register is slave dependent and will differ based on slave setup. In other words, it's just an example that you can write to a slave's register. In this example, Mbed OS acts as the SPI master, and DigitalOut acts as the slave chip select (cs).
+The following example uses the WHOAMI register. The WHOAMI register is an ID register of the slave device. In other words, it's just an example that you can write to a slave's register. In this example, Mbed OS acts as the SPI master, and DigitalOut acts as the slave chip select (cs).
 
 [![View code](https://www.mbed.com/embed/?url=https://os.mbed.com/teams/mbed_example/code/SPI_HelloWorld/)](https://os.mbed.com/teams/mbed_example/code/SPI_HelloWorld/file/3789302f33bc/main.cpp)