Update link in README.md

Sorry for the noise, links with markdown notation don't seem
to work in README.md.

README.md

@@ -11,4 +11,5 @@
     currently maintained by a small group of developers across
     the world with a central repository in the US.
 
-    For more information see [README.txt](https://github.com/fltk/fltk/blob/master/README.txt).
+    For more information see README.txt:
+    https://github.com/fltk/fltk/blob/master/README.txt