Add plugin snapshot repository to Data REST sample

Previously, if the main project hadn't been installed in the local
Maven repository (./gradlew install), rest-notes-spring-data-rest
would fail to build as the plugin dependency,
spring-restdocs-asciidoctor, was not available.

This commit corrects the problem by adding
repo.spring.io/libs-snapshot as a plugin repository in addition to
its existing declaration as a "normal" repository.

Fixes gh-382
Closes gh-383

samples/rest-notes-spring-data-rest/pom.xml

@@ -128,4 +128,15 @@
 		</repository>
 	</repositories>
 
+	<pluginRepositories>
+		<pluginRepository>
+			<id>spring-snapshots</id>
+			<name>Spring snapshots</name>
+			<url>https://repo.spring.io/libs-snapshot</url>
+			<snapshots>
+				<enabled>true</enabled>
+			</snapshots>
+		</pluginRepository>
+	</pluginRepositories>
+
 </project>