Add correct .txt extension to Changelog entry

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/ChangeLog.d/asn1write-0-fix b/ChangeLog.d/asn1write-0-fix.txt
similarity index 100%
rename from ChangeLog.d/asn1write-0-fix
rename to ChangeLog.d/asn1write-0-fix.txt