JiraLinkInlineParser.cs - Make digits in JiraKey's posible

src/Markdig/Extensions/JiraLinks/JiraLinkInlineParser.cs

@@ -41,7 +41,7 @@ public override bool Match(InlineProcessor processor, ref StringSlice slice)
  var endKey = slice.Start;
 
  //read as many uppercase characters as required - project key
- while (current.IsAlphaUpper())
+ while (current.IsAlphaUpper() || current.IsDigit())
  {
  endKey = slice.Start;
  current = slice.NextChar();