docs: Fix command reference for 'apdu' command

This fixes the below error during build of the documentation:

pysim/docs/shell.rst:349: ERROR: "<class 'pySim-shell.PySimCommands'>" has no attribute "apdu_cmd_parser"

Change-Id: If89b66a45ea18b5a3fc56bf77b05e679463da5a8
diff --git a/docs/shell.rst b/docs/shell.rst
index 73b988a..dea0bb4 100644
--- a/docs/shell.rst
+++ b/docs/shell.rst
@@ -342,7 +342,7 @@
 ~~~~
 .. argparse::
    :module: pySim-shell
-   :func: PySimCommands.apdu_cmd_parser
+   :func: PysimApp.apdu_cmd_parser