doc/opencl: speling and gramer fixes

Found-by: durandal_1707
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/opencl.texi

@@ -10,10 +10,12 @@ supported options follows:
 Set build options which used to compiled kernels, see reference "OpenCL Specification Version: 1.2 chapter 5.6.4"
 
 @item platform_idx
-Select platform to run OpenCL code, the platform_idx is the index of platform in device list which can be getted by function av_opencl_get_device_list().
+Select platform to run OpenCL code, the platform_idx is the index of platform
+in the device list which can be obtained with av_opencl_get_device_list().
 
 @item device_idx
-Select device to run OpenCL code, the device_idx is the index of device in device list which can be getted by function av_opencl_get_device_list().
+Select device to run OpenCL code, the device_idx is the index of device in
+the device list which can be obtained with av_opencl_get_device_list().
 
 @end table