Client OAuth2 properties to use scope not scopes

OAuth2ClientProperties.Registration (which captures .properties and
.yml for OAuth2 Client) has a member `scope` but not `scopes`. Samples
and documentation were using `scopes` and have now been updated to use
`scope`.

Fixes gh-6510

docs/manual/src/docs/asciidoc/_includes/reactive/oauth2/access-token.adoc

@@ -14,7 +14,7 @@ spring:
           github:
             client-id: replace-with-client-id
             client-secret: replace-with-client-secret
-            scopes: read:user,public_repo
+            scope: read:user,public_repo
 ----
 
 You will need to replace the `client-id` and `client-secret` with values registered with GitHub.

samples/boot/oauth2webclient-webflux/README.adoc

@@ -40,7 +40,7 @@ spring:
             client-id: replace-with-client-id
             client-secret: replace-with-client-secret
             provider: github
-            scopes: read:user,public_repo
+            scope: read:user,public_repo
 ----
 +
 .OAuth Client properties

samples/boot/oauth2webclient-webflux/src/main/resources/application.yml

@@ -16,6 +16,6 @@ spring:
             client-id: replace-with-client-id
             client-secret: replace-with-client-secret
             provider: github
-            scopes: read:user,public_repo
+            scope: read:user,public_repo
 
 resource-uri: https://api.github.com/user/repos

samples/boot/oauth2webclient/README.adoc

@@ -40,7 +40,7 @@ spring:
             client-id: replace-with-client-id
             client-secret: replace-with-client-secret
             provider: github
-            scopes: read:user,public_repo
+            scope: read:user,public_repo
 ----
 +
 .OAuth Client properties

samples/boot/oauth2webclient/src/main/resources/application.yml

@@ -16,6 +16,6 @@ spring:
             client-id: replace-with-client-id
             client-secret: replace-with-client-secret
             provider: github
-            scopes: read:user,public_repo
+            scope: read:user,public_repo
 
 resource-uri: https://api.github.com/user/repos