Update library version number to 2.1.15
diff --git a/doxygen/input/doc_mainpage.h b/doxygen/input/doc_mainpage.h
index 4e5c754..a44f02d 100644
--- a/doxygen/input/doc_mainpage.h
+++ b/doxygen/input/doc_mainpage.h
@@ -21,7 +21,7 @@
  */
 
 /**
- * @mainpage mbed TLS v2.1.14 source code documentation
+ * @mainpage mbed TLS v2.1.15 source code documentation
  *
  * This documentation describes the internal structure of mbed TLS.  It was
  * automatically generated from specially formatted comment blocks in