fix regex to react only on !ping or !ping <msg>

bot.ps1

@@ -234,7 +234,7 @@ function Open-JoinEvent {
     }
 
     #the "ball" is a string returned by the bot
-    if($Event.content.msgtype -eq 'm.text' -and $Event.content.body -match '^!ping( (?<ball>.*))?') {
+    if($Event.content.msgtype -eq 'm.text' -and $Event.content.body -match '^!ping($| (?<ball>.*))') {
         $difference = Measure-TimeDifference -OriginTime $origin_time
         $readable_timespan = ConvertTo-HumanReadableTimespan -TimeSpan $difference