Fix invalid logging call

[flaeppe]

• '.format' string syntax not supported by logging module

[codecov]

Codecov Report

Merging #9 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #9   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          14     14           
  Lines         713    719    +6     
  Branches       55     55           
=====================================
+ Hits          713    719    +6

Impacted Files	Coverage Δ
kapten/tool.py	100% <ø> (ø)	⬆️
tests/test_cli.py	100% <100%> (ø)	⬆️

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[flaeppe]

Client startup raised following silently:

 --- Logging error ---
 Traceback (most recent call last):
   File "/usr/local/lib/python3.8/logging/__init__.py", line 1081, in emit
     msg = self.format(record)
   File "/usr/local/lib/python3.8/logging/__init__.py", line 925, in format
     return fmt.format(record)
   File "/usr/local/lib/python3.8/logging/__init__.py", line 664, in format
     record.message = record.getMessage()
   File "/usr/local/lib/python3.8/logging/__init__.py", line 369, in getMessage
     msg = msg % self.args
 TypeError: not all arguments converted during string formatting
 Call stack:
   File "/app/venv/bin/kapten", line 11, in <module>
     load_entry_point('kapten==1.3', 'console_scripts', 'kapten')()
   File "/app/venv/lib/python3.8/site-packages/kapten/cli.py", line 114, in command
     loop.run_until_complete(client.healthcheck())
   File "/usr/local/lib/python3.8/asyncio/base_events.py", line 595, in run_until_complete
     self.run_forever()
   File "/usr/local/lib/python3.8/asyncio/base_events.py", line 563, in run_forever
     self._run_once()
   File "/usr/local/lib/python3.8/asyncio/base_events.py", line 1844, in _run_once
     handle._run()
   File "/usr/local/lib/python3.8/asyncio/events.py", line 81, in _run
     self._context.run(self._callback, *self._args)
   File "/app/venv/lib/python3.8/site-packages/kapten/tool.py", line 49, in healthcheck
     logger.info(
 Message: 'Tracking {} service{}'
 Arguments: (10, 's')