fix(spanner): tweak the tag name of a FGAC sample (#10266)
fix(spanner): tweak the tag name of a FGAC sample (#10266)

The sample specification document was ambiguous about whether one
of the FGAC tag names should be plural or not.  The one we chose
lost the toss.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp