fix broken link of datamodule

[cauliyang]

What does this PR do?

Fix broken link: https://pytorch-lightning.readthedocs.io/en/latest/extensions/datamodules.html
Now: https://pytorch-lightning.readthedocs.io/en/latest/data/datamodule.html

Before submitting

• Did you make sure title is self-explanatory and the description concisely explains the PR?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you list all the breaking changes introduced by this pull request?
• Did you test your PR locally with pytest command?
• Did you run pre-commit hooks with pre-commit run -a command?

Did you have fun?

Sure!

[codecov-commenter]

Codecov Report

Base: 84.81% // Head: 84.81% // No change to project coverage 👍

Coverage data is based on head (601f3c7) compared to base (f6147fa).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #444   +/-   ##
=======================================
  Coverage   84.81%   84.81%           
=======================================
  Files           9        9           
  Lines         349      349           
=======================================
  Hits          296      296           
  Misses         53       53           

Impacted Files	Coverage Δ
src/datamodules/mnist_datamodule.py	94.73% <ø> (ø)

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[KLOSYX]

The old url is invalid.

[ashleve]

@cauliyang Thank you for the fix!