style: update astyle version

3.1 is the version available in Ubuntu 20.04LTS, which is recommended in the
current install instructions. Later versions also work and output seems to be
stable for our sources. Versions earlier than 3.0 no longer work with the
options in our .astylerc.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

processes/style-guide.md

@@ -35,7 +35,7 @@ Code should compile without warnings, with `-Wall`.
 ### Automatic formatting
 
 For automatic formatting of C code, we use
-[astyle](http://astyle.sourceforge.net/), version 2.05.1, with the
+[astyle](http://astyle.sourceforge.net/), version 3.1, with the
 settings declared in our
 [astylerc](https://github.com/seL4/seL4_tools/blob/master/misc/astylerc).