fix: use correct quickstart URL

[devjgm]

This change is 

[codecov]

Codecov Report

Merging #1339 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #1339 +/- ## ========================================== + Coverage 94.64% 94.64% +<.01%  ========================================== Files 172 173 +1 Lines 13630 13639 +9 ========================================== + Hits 12900 12909 +9  Misses 730 730

Impacted Files	Coverage Δ
google/cloud/spanner/client.cc	97.22% <0%> (-0.7%)	⬇️
...on_tests/rpc_failure_threshold_integration_test.cc	85.55% <0%> (-0.16%)	⬇️
google/cloud/spanner/value.h	92.8% <0%> (-0.11%)	⬇️
google/cloud/spanner/keys.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)	⬆️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	91.97% <0%> (ø)	⬆️
google/cloud/spanner/internal/spanner_stub.cc	66.66% <0%> (ø)	⬆️
google/cloud/spanner/transaction.h	100% <0%> (ø)	⬆️
google/cloud/spanner/instance_admin_connection.h	100% <0%> (ø)
... and 2 more

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