Update HISTORY.txt

Author: petecooper

HISTORY.txt

@@ -3,7 +3,7 @@ Changes in 4.7.3
 * Fixed: 'Export to disk' checkbox state inconsistencies for new assets.
 * Fixed: Logout issue in Classic admin theme.
 * Fixed: A bug when PHP 'mbstring' extension is disabled.
-* Fixed: Prevent Alt+s from triggering save (thanks, o10154016).
+* Fixed: Prevent Alt+S from triggering save (thanks, o10154016).
 * Fixed: Language .ini files trump old .txt files.
 * Changed: Permit falling back on pophelp in 'lang' database table.
 * Added: <txp:yield /> values are available in external forms too.