docs: fix tool versions hook example

[cbachhuber]

Yaml recognizes value 3.12 as float, not str:

from ruamel.yaml import YAML
yaml = YAML(typ="safe")
config=r"""name: tool-versions
sync_versions:
  - name: python
    version: 3.12
    entries:
      - path: .python-version
        pattern: ([0-9]+\.[0-9]+)
"""
data = yaml.load(config)
type(data['sync_versions'][0]['version'])  # prints '<class 'float'>'

This causes dev_tools/sync_tool_versions.py to raise

  Error: invalid config in /home/chris/improvements/.versions.yaml: Each sync_versions entry must have a non-empty 'version' in /home/chris/improvements/.versions.yaml

with the config from the example. This doesn't hint to the root problem. Thus, improve the error message and fix the example in the docs. Alternatively, we could convert float to str to simplify the config file.

Btw:

>>> from ruamel.yaml import YAML
>>> yaml = YAML(typ="safe")
>>> type(yaml.load("version: 1.91.0")['version'])
<class 'str'>