IVGCVSW-6212 Search bar on github in doxygen docu not working

* Update HTML_FILE_EXTENSION to html

Signed-off-by: Nikhil Raj <nikhil.raj@arm.com>
Change-Id: I7b4107007378a37ef470087e977dd0aaeae2f0cc
diff --git a/docs/Doxyfile b/docs/Doxyfile
index b163241..0a08f3f 100644
--- a/docs/Doxyfile
+++ b/docs/Doxyfile
@@ -1161,7 +1161,7 @@
 # The default value is: .html.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_FILE_EXTENSION    = .xhtml
+HTML_FILE_EXTENSION    = .html
 
 # The HTML_HEADER tag can be used to specify a user-defined HTML header file for
 # each generated HTML page. If the tag is left blank doxygen will generate a