Fixed grammar in comments.

examples/using_custom_kernels_ex.cpp

 // The contents of this file are in the public domain. See LICENSE_FOR_EXAMPLE_PROGRAMS.txt
 /*
 
-    This is an example showing how to defined custom kernel functions for use with 
+    This is an example showing how to define custom kernel functions for use with 
     the machine learning tools in the dlib C++ Library.
 
     This example assumes you are somewhat familiar with the machine learning
@@ -80,7 +80,7 @@ struct ukf_kernel
 
 /*
     Here we define serialize() and deserialize() functions for our new kernel.  Defining
-    these function is optional.  However, if you don't define them you won't be able
+    these functions is optional.  However, if you don't define them you won't be able
     to save your learned decision_function objects to disk. 
 */