chore: regenerate README

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

2021-03-18 03:30:17,880 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-18 03:30:18,802 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

Full log will be available here:
https://source.cloud.google.com/results/invocations/fe9a1706-993a-460d-ac40-aaa3b0bb367f/targets

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[yoshi-approver]

Rubber stamped PR!

[codecov]

Codecov Report

Merging #991 (92be423) into master (8eab323) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #991      +/-   ##
============================================
- Coverage     85.25%   85.23%   -0.03%     
+ Complexity     2662     2660       -2     
============================================
  Files           146      146              
  Lines         14404    14404              
  Branches       1395     1395              
============================================
- Hits          12280    12277       -3     
- Misses         1541     1542       +1     
- Partials        583      585       +2     

Impacted Files	Coverage Δ	Complexity Δ
...ud/spanner/SessionPoolAsyncTransactionManager.java	85.71% <0.00%> (-1.51%)	12.00% <0.00%> (-2.00%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.07% <0.00%> (-0.20%)	73.00% <0.00%> (ø%)
...m/google/cloud/spanner/connection/SpannerPool.java	87.89% <0.00%> (+0.52%)	33.00% <0.00%> (ø%)

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