Bugfix: call get ApiFuture object to prevent RejectedExecutionException

[mannuscript]

Fixes #6784

[codecov]

Codecov Report

Merging #6787 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #6787      +/-   ##
============================================
- Coverage     46.37%   46.37%   -0.01%     
- Complexity    28026    28028       +2     
============================================
  Files          2614     2615       +1     
  Lines        288327   288309      -18     
  Branches      33810    33809       -1     
============================================
- Hits         133723   133710      -13     
+ Misses       144339   144334       -5     
  Partials      10265    10265

Impacted Files	Coverage Δ	Complexity Δ
...oogle/cloud/spanner/jdbc/SingleUseTransaction.java	86% <0%> (-1%)	36% <0%> (-1%)
.../google/cloud/spanner/jdbc/CredentialsService.java	90.47% <0%> (-0.64%)	8% <0%> (-3%)
...rc/main/java/com/google/cloud/storage/Storage.java	79.39% <0%> (ø)	0% <0%> (ø)	⬇️
...google/cloud/logging/logback/MDCEventEnhancer.java	83.33% <0%> (ø)	4% <0%> (?)
.../google/cloud/logging/logback/LoggingAppender.java	68.5% <0%> (+4.72%)	30% <0%> (+2%)	⬆️

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[hannahrogers-google]

Hi @mannuscript, thanks for bringing this to our attention! And thanks for creating this PR. I agree that this example should definitely be updated. However, we would like to change this example so that it matches other Publish examples that we give in the library.

I am going to close this PR for now, and will update the README examples soon. Thanks!