chore: regenerate README

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

2021-04-15 23:44:42,888 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-04-15 23:44:44,180 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

Full log will be available here:
https://source.cloud.google.com/results/invocations/96a43dab-8200-4ece-bd99-37de6ea05cd8/targets

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[yoshi-approver]

Rubber stamped PR!

[codecov]

Codecov Report

Merging #1070 (639fce6) into master (7992342) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1070   +/-   ##
=========================================
  Coverage     85.25%   85.25%           
- Complexity     2660     2661    +1     
=========================================
  Files           155      155           
  Lines         14448    14448           
  Branches       1362     1362           
=========================================
  Hits          12317    12317           
  Misses         1564     1564           
  Partials        567      567           

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/spanner/SessionPool.java	88.93% <0.00%> (-0.20%)	71.00% <0.00%> (-1.00%)
...ud/spanner/SessionPoolAsyncTransactionManager.java	87.21% <0.00%> (+1.50%)	14.00% <0.00%> (+2.00%)

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