Fix Config Documentation

We use spanner-refdb.config, not gerrit.config

Change-Id: If5630cb385964bb64b5c28d3cadd4764fceb846e

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 61feccf..45c3507 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -1,7 +1,7 @@
 Configuration
 =========================
 
-The plugin can be configured as usual in the gerrit.config.
+The plugin configuration belongs in spanner-refdb.config.
 The credentialsPath is required when not using the Spanner emulator.
 
 ```