doc/sphinx: fix Python string escapes

Python 3.6 introduced a DeprecationWarning for invalid escape sequences.
This is upgraded to a SyntaxWarning in Python 3.12, and will eventually
be a syntax error.

Fix these now to get ahead of it before it's an error.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Message-ID: <20230912060801.95533-3-bgray@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Adapted for U-Boot
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>