docs(lightning): enhance hint for epoch-level logging

[adosar]

What does this PR do?

Improves the documentation for logging a metric only on epoch-level.

In the docs we find the following hint:

When using any Modular metric, calling self.metric(...) or self.metric.forward(...) serves the dual purpose
of calling self.metric.update() on its input and simultaneously returning the metric value over the provided
input. So if you are logging a metric only on epoch-level (as in the example above), it is recommended to call
self.metric.update() directly to avoid the extra computation.

The original documentation snippet is intended to illustrate logging a metric only on epoch-level. However, the snippet sets on_step=True, which contradicts the stated motivation. Changed it to on_step=False to ensure the example correctly demonstrates logging only on epoch-level.

Fixes #<issue_number>

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--3282.org.readthedocs.build/en/3282/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37%. Comparing base (122a852) to head (e1141d2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #3282    +/-   ##
=======================================
- Coverage      37%     37%    -1%     
=======================================
  Files         364     349    -15     
  Lines       20096   19901   -195     
=======================================
- Hits         7520    7326   -194     
+ Misses      12576   12575     -1     

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