docs: homogenize region tags

[Strykrol]

These samples' region tags slightly differ from identical snippets in different languages. The following is the remedy:

1. Merge this PR to fix GitHub source (keep old tag for now)
2. Fix doc code includes to represent the better region tag
3. Submit a second PR to clean-up redundant region tag wrapping

[snippet-bot]

Here is the summary of changes.

You added 1 region tag.

• samples/snippets/src/main/java/com/example/spanner/SpannerSample.java:1625, tag spanner_cancel_backup_create

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[codecov]

Codecov Report

Merging #752 (7fce014) into master (daef458) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #752      +/-   ##
============================================
- Coverage     85.15%   85.15%   -0.01%     
+ Complexity     2564     2563       -1     
============================================
  Files           142      142              
  Lines         13952    13960       +8     
  Branches       1331     1331              
============================================
+ Hits          11881    11887       +6     
- Misses         1512     1514       +2     
  Partials        559      559              

Impacted Files	Coverage Δ	Complexity Δ
.../google/cloud/spanner/SpannerExceptionFactory.java	82.47% <0.00%> (-2.07%)	46.00% <0.00%> (-1.00%)
...m/google/cloud/spanner/spi/v1/GapicSpannerRpc.java	83.39% <0.00%> (+0.17%)	81.00% <0.00%> (ø%)

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