Fix .ts heuristic rule (#3171)

Use the opening TS tag instead of the closing tag since
the closing tag might be at the end of the file,
after the max number of lines we read with a LazyBlob

lib/linguist/heuristics.rb

@@ -430,7 +430,7 @@ module Linguist
     end
 
     disambiguate ".ts" do |data|
-      if data.include?("</TS>")
+      if data.include?("<TS")
         Language["XML"]
       else
         Language["TypeScript"]