fix(spanner): indent code example for Encoder and Decoder (#4128)
fix(spanner): indent code example for Encoder and Decoder (#4128)

Co-authored-by: Hengfeng Li <b0a1bcb38cb78116cc20da5a7cacec7b885825f4@google.com>
googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go