doc/scripts: use raw string for regex in gen_state_diagram.py

doc/scripts: use raw string for regex in gen_state_diagram.py

Python 3.12 emits SyntaxWarning for invalid escape sequences in ordinary
string literals. The re.search() call on line 162 was the only pattern
in the file passed as a non-raw string, causing:

  doc/scripts/gen_state_diagram.py:162: SyntaxWarning: invalid escape
    sequence '\s'
    i = re.search("return\s+transit<\s*(\w*)\s*>()", line)

Add the r"" prefix to match the other re.search / re.finditer / re.sub
call sites in the same file. No behavior change; \s was already being
interpreted as a regex whitespace class because Python leaves unknown
escapes untouched, but this will become a SyntaxError in a future
Python release.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>