Rename label variable to circumvent link issue in docs.

[mkcor]

Description

Fixes #5414 -- thanks for catching this, @jni

Another way of avoiding this issue is to import submodules rather than functions; the link is correct when calling measure.label. Personally, I prefer writing

from skimage import draw, measure, transform

in gallery examples, so functions are prefixed with their submodule and the reader becomes familiar with the code structure...

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

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[pep8speaks]

Hello @mkcor! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file doc/examples/segmentation/plot_regionprops.py:

Line 86:1: E402 module level import not at top of file

[grlee77]

This looks good. The failures seen here are unrelated and resolved by #5433