Update reference to the latest PDF manual.

Author: objecthub

README.md

@@ -30,9 +30,9 @@ outperform the MacBook Pro 16" (2019) for simple LispKit benchmarks.
 
 [_LispPad_](http://www.lisppad.app) implements a simple, lightweight, integrated
 development environment for _LispKit_ on macOS with a Cocoa-based UI. The
-[LispPad Library Reference](https://zenger.org/lisppad/assets/LispPad_Reference_2.0.1.pdf)
+[LispPad Library Reference](https://zenger.org/lisppad/assets/LispPad_Reference_2.1.pdf)
 documents the core _LispPad_ and _LispKit_ libraries in
-[PDF](https://zenger.org/lisppad/assets/LispPad_Reference_2.0.1.pdf) form. On iOS,
+[PDF](https://zenger.org/lisppad/assets/LispPad_Reference_2.1.pdf) form. On iOS,
 application [_LispPad Go_](https://apps.apple.com/us/app/lisppad-go/id1565747728) provides a simple
 Scheme IDE based on LispKit. The [source code of _LispPad Go_](https://github.com/objecthub/swift-lisppad-ios)
 is available on GitHub. A much simpler command-line tool for iOS is included in the LispKit