Update README.md

#5 correct error of mark-down.

Author: tanakaryo

Diff for: infrastructure-as-code/terraform/README.md

@@ -1,3 +1,3 @@
 # This is Terraform sample source on Google Cloud.
  - `create-single-alloydb` is use only alloydb.
- - 'create-single-spanner' is use only cloud spanner.
+ - `create-single-spanner` is use only cloud spanner.