chore: regenerate README

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

2021-04-13 09:11:35,444 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-04-13 09:11:36,730 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

Full log will be available here:
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[codecov]

Codecov Report

Merging #1059 (74d2d38) into master (a54ee40) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1059   +/-   ##
=========================================
  Coverage     85.25%   85.25%           
- Complexity     2658     2660    +2     
=========================================
  Files           155      155           
  Lines         14453    14453           
  Branches       1362     1362           
=========================================
  Hits          12322    12322           
+ Misses         1564     1562    -2     
- Partials        567      569    +2     

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/spanner/SessionPool.java	89.12% <0.00%> (ø)	73.00% <0.00%> (+1.00%)
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	18.00% <0.00%> (+1.00%)

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