chore: regenerate README

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

2021-04-16 15:04:26,019 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-secretmanager/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-04-16 15:04:27,324 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

Full log will be available here:
https://source.cloud.google.com/results/invocations/66c07b0e-ee3f-4070-8cda-e3572eccfb9d/targets

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[yoshi-approver]

Rubber stamped PR!

[codecov]

Codecov Report

Merging #447 (fb44809) into master (75a0ade) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #447   +/-   ##
=========================================
  Coverage     81.52%   81.52%           
  Complexity      256      256           
=========================================
  Files            12       12           
  Lines          1700     1700           
  Branches         22       22           
=========================================
  Hits           1386     1386           
  Misses          280      280           
  Partials         34       34           

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