Update README.md (#820)

Typo in the readme file.

Author: sheikzayed

hello-jniCallback/README.md

@@ -1,6 +1,6 @@
 Hello JNI Callback
 ==================
-This sample is an Aadd-on to Hello JNI sample to demonstrate calling back to Java from C code
+This sample is an add-on to Hello JNI sample to demonstrate calling back to Java from C code
 - create a java class instance from C code
 - call java class static and non-static member functions