docs: update Pub/Sub publisher docstring example

[anguillanneuf]

Fixes#3920

Missed the docstring in a previous PR #4120 that addressed the same issue.

[anguillanneuf]

@chingor13 Do you know why the test kept failing?

[chingor13]

It looks like a flakey spanner JDBC test. @kolea2, @olavloite might know more.

[kolea2]

Sorry about that, we're tracking #6075 to fix this. I've rerun the build.

[codecov]

Codecov Report

Merging #6100 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #6100 +/- ##
============================================
+ Coverage 47.38% 47.4% +0.01% - Complexity 27180 27228 +48 
============================================
Files 2523 2523 Lines 274580 274830 +250 Branches 31380 31391 +11 ============================================
+ Hits 130118 130289 +171 - Misses 134852 134928 +76 - Partials 9610 9613 +3

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	89.11% <ø> (ø)	40 <0> (ø)	⬇️
...gtable/admin/v2/BigtableInstanceAdminSettings.java	97.77% <0%> (-2.23%)	11% <0%> (+5%)
...e/cloud/vision/v1p4beta1/ImageAnnotatorClient.java	60.56% <0%> (-1.44%)	25% <0%> (+6%)
.../bigtable/admin/v2/BigtableTableAdminSettings.java	73.25% <0%> (+1.51%)	14% <0%> (+7%)	⬆️
...e/cloud/bigtable/data/v2/BigtableDataSettings.java	37.2% <0%> (+2.42%)	4% <0%> (+1%)	⬆️
...d/dialogflow/v2beta1/SessionEntityTypesClient.java	64.77% <0%> (+4.99%)	39% <0%> (+16%)	⬆️
.../cloud/dialogflow/v2/SessionEntityTypesClient.java	66.02% <0%> (+6.24%)	36% <0%> (+13%)	⬆️

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