Fix command-buffer extension typo (#780)

The return type of `clCreateCommandBufferKHR` is
missing the `_khr` suffix in its definition.

Author: Ewan Crawford

ext/cl_khr_command_buffer.asciidoc

@@ -481,7 +481,7 @@ The function
 indexterm:[clCreateCommandBufferKHR]
 [source]
 ----
-cl_command_buffer clCreateCommandBufferKHR(
+cl_command_buffer_khr clCreateCommandBufferKHR(
     cl_uint num_queues,
     const cl_command_queue* queues,
     const cl_command_buffer_properties_khr* properties,