minor grammar fix

Author: bashbaug

api/opencl_platform_layer.asciidoc

@@ -447,7 +447,7 @@ include::{generated}/api/version-notes/CL_DEVICE_TYPE_ACCELERATOR.asciidoc[]
 
 include::{generated}/api/version-notes/CL_DEVICE_TYPE_CUSTOM.asciidoc[]
   | Specialized devices that support a subset of the OpenCL runtime APIs for
-    directed tasks but is not OpenCL conformant.
+    directed tasks but are not OpenCL conformant.
     A custom device must implement all of the OpenCL runtime APIs, but may
     return implementation-defined error codes for unsupported functionality.