docs/pyboard: Fix typo in pyb.Switch tutorial.

docs/pyboard/tutorial/switch.rst

@@ -93,7 +93,7 @@ on the pin for any changes, and the following will occur:
    running Python script.
 3. The microcontroller starts executing the special interrupt handler
    associated with the switch's external trigger.  This interrupt handler
-   get the function that you registered with ``sw.callback()`` and executes
+   gets the function that you registered with ``sw.callback()`` and executes
    it.
 4. Your callback function is executed until it finishes, returning control
    to the switch interrupt handler.