Fix broken link in docs

[htbkoo]

Which issue(s) does this Pull Request fix?

N/A
(This is a very simple fix for the documentation so I had not raised the corresponding issue - please let me know if I should raise that first)

Details of the Pull Request

Updated docs/py/structured_data_regression.py by fixing the link to the California housing dataset hosted on scikit-learn as the current link is no longer accessible

[htbkoo]

Hi Keras team,

First of all, thanks a lot for your efforts in creating / maintain this fantastic library!

FYI, this is my very first PR created at this repo so please do let me know if there is anything wrong with this PR and I would try my best to fix that!

Thanks!

[htbkoo]

FYI I had tried to do a very simple quick search at the repo and apparently these 2 places are the only references to the example data sets hosted on scikit-learn.org

[haifeng-jin]

Thanks for the PR!

[codecov]

Codecov Report

Merging #1618 (86f82ca) into master (e503c51) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1618   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           51        51           
  Lines         3339      3339           
=========================================
  Hits          3339      3339           

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