chore: regenerate README

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

2021-02-24 00:58:30,337 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-02-24 00:58:32,050 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

Full log will be available here:
https://source.cloud.google.com/results/invocations/9377e948-e2f4-435f-9c12-01d63cf37ec4/targets

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[yoshi-approver]

Rubber stamped PR!

[codecov]

Codecov Report

Merging #908 (76ce5f8) into master (e3a5342) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #908      +/-   ##
============================================
- Coverage     85.23%   85.18%   -0.06%     
+ Complexity     2627     2621       -6     
============================================
  Files           145      145              
  Lines         14287    14287              
  Branches       1379     1379              
============================================
- Hits          12178    12170       -8     
- Misses         1533     1542       +9     
+ Partials        576      575       -1     

Impacted Files	Coverage Δ	Complexity Δ
...r/AbortedDueToConcurrentModificationException.java	42.85% <0.00%> (-42.86%)	1.00% <0.00%> (-1.00%)
...m/google/cloud/spanner/connection/FailedQuery.java	75.86% <0.00%> (-3.45%)	2.00% <0.00%> (-1.00%)
.../google/cloud/spanner/SpannerExceptionFactory.java	84.68% <0.00%> (-0.91%)	47.00% <0.00%> (-1.00%)
...m/google/cloud/spanner/connection/SpannerPool.java	87.36% <0.00%> (-0.53%)	33.00% <0.00%> (ø%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.07% <0.00%> (-0.20%)	72.00% <0.00%> (-1.00%)
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	17.00% <0.00%> (-1.00%)

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