doc/utils: use @code markup for opencl function names.

doc/utils.texi

@@ -579,13 +579,13 @@ See reference "OpenCL Specification Version: 1.2 chapter 5.6.4".
 Select the index of the platform to run OpenCL code.
 
 The specified index must be one of the indexes in the device list
-which can be obtained with av_opencl_get_device_list().
+which can be obtained with @code{av_opencl_get_device_list()}.
 
 @item device_idx
 Select the index of the device used to run OpenCL code.
 
 The specifed index must be one of the indexes in the device list which
-can be obtained with av_opencl_get_device_list().
+can be obtained with @code{av_opencl_get_device_list()}.
 
 @end table