Fix EventQueue API tutorial bugs

[yennster]

Fixed EventQueue API tutorial bugs

• Updated events_ex_1 here: https://os.mbed.com/teams/mbed_example/code/events_ex_1/file/69c11c7877b6/main.cpp/

[geky]

@yennster, it looks like the added printf (in the fall handler) will run in irq context?

[yennster]

@geky Yes, that's because this tutorial is weird in that it shows you the incorrect way to use interrupt handlers first, and then explains how you fix it.

If you read through the tutorial from top to bottom you will see why the example needs to be "wrong"

[geky]

Hmmm, ok, I think we changed before because ironically it was wrong