Fix preversion.sh to stage History.md

This fixes an issue that was introduced in [1], where Changelog.txt was
renmaed to History.md, without updating this script to stage the changed
file with then new filename.

[1] e9f40a2

scripts/preversion.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 echo 'Updating History.md'
 git changelog --no-merges
-git add Changelog.txt
+git add History.md
 
 echo 'Updating AUTHORS'
 git authors --list > AUTHORS