chore: regenerate README

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

2021-02-23 22:33:45,447 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-02-23 22:33:47,224 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

Full log will be available here:
https://source.cloud.google.com/results/invocations/6af050d2-f043-4217-a10e-c52d9cc501b8/targets

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[yoshi-approver]

Rubber stamped PR!

[codecov]

Codecov Report

Merging #905 (2038bbe) into master (2e0ebed) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #905      +/-   ##
============================================
+ Coverage     85.18%   85.21%   +0.02%     
- Complexity     2622     2624       +2     
============================================
  Files           145      145              
  Lines         14287    14287              
  Branches       1379     1379              
============================================
+ Hits          12171    12174       +3     
+ Misses         1539     1537       -2     
+ Partials        577      576       -1     

Impacted Files	Coverage Δ	Complexity Δ
...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	89.26% <0.00%> (+0.19%)	73.00% <0.00%> (+1.00%)
.../google/cloud/spanner/SpannerExceptionFactory.java	86.48% <0.00%> (+1.80%)	48.00% <0.00%> (+1.00%)

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