api: mraa: gpio: Fix doxygen warning
api: mraa: gpio: Fix doxygen warning

This commit fixes the below doxygen warning:

"warning: unexpected token in comment block while parsing the argument of command param"

Signed-off-by: Manivannan Sadhasivam <7cdffa2a61f6c51ff0ae24b4793b8c7f31d1b1b0@linaro.org>
Signed-off-by: Brendan Le Foll <3e0e8c4069934e8b47d99704927a85c66c0ceb33@intel.com>

stefan-andritoiu/mraa,arfoll/mraa,intel-iot-devkit/mraa,malikabhi05/mraa,alext-mkrs/mraa,alext-mkrs/mraa,malikabhi05/mraa,yongli3/mraa,malikabhi05/mraa,arfoll/mraa,malikabhi05/mraa,stefan-andritoiu/mraa,stefan-andritoiu/mraa,alext-mkrs/mraa,KurtE/mraa,malikabhi05/mraa,Propanu/mraa,g-vidal/mraa,intel-iot-devkit/mraa,yongli3/mraa,KurtE/mraa,stefan-andritoiu/mraa,g-vidal/mraa,alext-mkrs/mraa,yongli3/mraa,g-vidal/mraa,intel-iot-devkit/mraa,g-vidal/mraa,yongli3/mraa,intel-iot-devkit/mraa,KurtE/mraa,yongli3/mraa,g-vidal/mraa,Propanu/mraa,intel-iot-devkit/mraa,arfoll/mraa,Propanu/mraa,arfoll/mraa,Propanu/mraa,arfoll/mraa,alext-mkrs/mraa,KurtE/mraa,KurtE/mraa,stefan-andritoiu/mraa,Propanu/mraa