Minor doc fix: add missing print statement in the plot_segmentations.py example

[AdeelH]

Description

The example prints out the number of generated segments for each of the four segmentation algorithms in the example except for Watershed. This PR fixes that oversight.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8
• Descriptive commit messages (see below)

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[grlee77]

Thanks, @AdeelH! looks good to me

[lagru]

Thanks! :)