doc: link existing example for ExecuteBatchDml

[coryan]

This change is 

[codecov]

Codecov Report

Merging #1395 into master will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1395      +/-   ##
==========================================
- Coverage   94.48%   94.32%   -0.16%     
==========================================
  Files         190      191       +1     
  Lines       15495    15536      +41     
==========================================
+ Hits        14640    14654      +14     
- Misses        855      882      +27

Impacted Files	Coverage Δ
google/cloud/spanner/samples/samples.cc	88.68% <ø> (+0.23%)	⬆️
google/cloud/spanner/client.h	100% <ø> (ø)	⬆️
google/cloud/spanner/internal/log_wrapper.h	71.42% <0%> (-13.19%)	⬇️
google/cloud/spanner/internal/session_pool.cc	82.81% <0%> (-12.98%)	⬇️
google/cloud/spanner/results.h	95.83% <0%> (-4.17%)	⬇️
...gle/cloud/spanner/internal/async_retry_unary_rpc.h	57.14% <0%> (-2.44%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	80.55% <0%> (-1.86%)	⬇️
google/cloud/spanner/internal/session.h	100% <0%> (ø)	⬆️
google/cloud/spanner/internal/transaction_impl.h	100% <0%> (ø)	⬆️
...cloud/spanner/internal/partial_result_set_resume.h	100% <0%> (ø)	⬆️
... and 6 more

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