Fix generate-prdoc for single audiences (paritytech#6103)

`prdoc check -n NNNN` (used in CI) enforces that yaml lists only be used
for lists with length more than one.

Previously this script generated a list no matter how many audiences
were provided, which generated a broken PRDoc when that number was 1.

Reproducing the issue locally:
```
python .github/scripts/generate-prdoc.py --pr 6022 --bump patch --audience runtime_user --force
prdoc check -n 6022
```
Fails on master but passes with this change

Co-authored-by: Bastian Köcher <[email protected]>

.github/scripts/generate-prdoc.py

@@ -122,14 +122,16 @@ def setup_parser(parser=None, pr_required=True):
 	return parser
 
 def snake_to_title(s):
-    return ' '.join(word.capitalize() for word in s.split('_'))
+	return ' '.join(word.capitalize() for word in s.split('_'))
 
 def main(args):
 	print(f"Args: {args}, force: {args.force}")
 	setup_yaml()
 	try:
 		# Convert snake_case audience arguments to title case
 		mapped_audiences = [snake_to_title(a) for a in args.audience]
+		if len(mapped_audiences) == 1:
+			mapped_audiences = mapped_audiences[0]
 		from_pr_number(args.pr, mapped_audiences, args.bump, args.force)
 		return 0
 	except Exception as e: