Fix Lua syntax highlighting.

Not defining langDef.mSingleLineComment was causing the entirety
of a Lua program to be seen as a comment.
diff --git a/TextEditor.cpp b/TextEditor.cpp
index 1492dcd..9b13b12 100644
--- a/TextEditor.cpp
+++ b/TextEditor.cpp
@@ -2718,6 +2718,7 @@
 

 		langDef.mCommentStart = "--[[";

 		langDef.mCommentEnd = "]]";

+		langDef.mSingleLineComment = "--";

 

 		langDef.mCaseSensitive = true;

 		langDef.mAutoIndentation = false;