Remove unnecessary line in DigitalIn.md

Remove extra words that don't add anything.

Author: Amanda Butler

docs/api/drivers/DigitalIn.md

@@ -6,8 +6,6 @@ You can use any of the numbered Arm Mbed pins can be used as a DigitalIn.
 
 ## DigitalIn class reference
 
-API summary
-
 [![View code](https://www.mbed.com/embed/?type=library)](https://os.mbed.com/docs/development/mbed-os-api-doxy/classmbed_1_1_digital_in.html)
 
 ## DigitalIn hello, world