refer to py3.10 in instructions, now that 3.9 is unsupported

Signed-off-by: Brian S. Stephan <bss@incorporeal.org>

README.md

@@ -7,8 +7,8 @@ A lightweight static site generator for Markdown-based sites.
 Something like the following should suffice:
 
 ```
-% virtualenv --python=python3.9 env-py3.9
+% virtualenv --python=python3.10 env-py3.10
-% source env-py3.9/bin/activate
+% source env-py3.10/bin/activate
 % pip install -U pip
 % pip install incorporeal-cms
 % incorporealcms-build ./path/to/instance ./path/to/output/www/root