Fix create_release.py script to handle bad underlines

The script was assuming the underline was the same length as the title
itself.

e.g. it did not handle.

```
1.3.4 (in development)
-------------------
```

See emscripten-core#24516

Author: sbc100

tools/maint/create_release.py

@@ -27,8 +27,10 @@ def update_changelog(release_version, new_version):
   changelog = utils.read_file(changelog_file)
   marker = f'{release_version} (in development)'
   pos = changelog.find(marker)
-  assert pos != -1
-  pos += 2 * len(marker) + 1
+  pos += len(marker) + 1
+  # Skip the next line which should just be hypens
+  assert(changelog[pos] == '-')
+  pos = changelog.find('\n', pos)
 
   # Add new entry
   today = datetime.now().strftime('%m/%d/%y')