typos in Introduction.md corrected

Introduction.md

@@ -399,4 +399,4 @@ grammar specification, which would render the grammar less readable.
 Now that you have seen how DHParser basically works, it is time to go
 through the process of desining and testing a domain specific notation
 step by step from the very start. Head over to the documentation in
-subdirectory and read the step by step guide.
\ No newline at end of file
+subdirectory and read the step by step guide.