Bump version to 2.28.3

Command used: ./scripts/bump_version.sh --version 2.28.3

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/doxygen/input/doc_mainpage.h b/doxygen/input/doc_mainpage.h
index 54b2f7a..4c76246 100644
--- a/doxygen/input/doc_mainpage.h
+++ b/doxygen/input/doc_mainpage.h
@@ -22,7 +22,7 @@
  */
 
 /**
- * @mainpage mbed TLS v2.28.2 source code documentation
+ * @mainpage mbed TLS v2.28.3 source code documentation
  *
  * This documentation describes the internal structure of mbed TLS.  It was
  * automatically generated from specially formatted comment blocks in