Readme: update OpenWrt version in Docker instructions

[ilario]

Simply updating the OpenWrt version in the instructions for compiling an image using Docker. Tested and the image gets created.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.95%. Comparing base (3b2ccf5) to head (4f24bbb).
Report is 28 commits behind head on master.

❗ Current head 4f24bbb differs from pull request most recent head bb0b5a2

Please upload reports for the commit bb0b5a2 to get more accurate results.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1116      +/-   ##
==========================================
+ Coverage   79.51%   79.95%   +0.44%     
==========================================
  Files          53       52       -1     
  Lines        4599     4441     -158     
==========================================
- Hits         3657     3551     -106     
+ Misses        942      890      -52     

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