Updated the missing APL books (#3495)

* Updated the missing APL books

* Removed the trailing slash

free-programming-books.md

@@ -841,8 +841,8 @@ Kerridge (PDF) (email address *requested*, not required)
 
 ### APL
 
-* [A Practical Introduction to APL1 & APL2](http://aplwiki.com/BooksAndPublications#A_Practical_Introduction_to_APL1_.26_APL2)
-* [A Practical Introduction to APL3 & APL4](http://aplwiki.com/BooksAndPublications#A_Practical_Introduction_to_APL3_.26_APL4)
+* [A Practical Introduction to APL1 & APL2](http://robertson.uk.net) - Graeme Donald Robertson (PDF)
+* [A Practical Introduction to APL3 & APL4](http://robertson.uk.net) - Graeme Donald Robertson (PDF)
 * [Mastering Dyalog APL](http://www.dyalog.com/mastering-dyalog-apl.htm) (PDF)