Fix snapshot build instructions

The previous code snippet contained code which would go in the `distributionManagement` section of a POM, meant for where to publish snapshots. It is however intended for users to use in their POMs to access published snapshot versions of the Spring Boot Admin artifacts. This change makes that more clear and gives the correct snippet to copy-and-paste into one's POM.

Author: shakuzen

README.md

@@ -101,13 +101,19 @@ Spring, Spring Boot and Spring Cloud are trademarks of [Pivotal Software, Inc.](
 *View history of registered applications*
 
 ## Snapshot builds
-You can access snapshot builds from the sonatype repository:
+You can access snapshot builds from the sonatype snapshot repository by adding the following to your `repositories`:
 ```xml
-<snapshotRepository>
- <id>sonatype-nexus-snapshots</id>
- <name>Sonatype Nexus Snapshots</name>
- <url>https://oss.sonatype.org/content/repositories/snapshots/</url>
-</snapshotRepository>
+<repository>
+	<id>sonatype-nexus-snapshots</id>
+	<name>Sonatype Nexus Snapshots</name>
+	<url>https://oss.sonatype.org/content/repositories/snapshots/</url>
+	<snapshots>
+		<enabled>true</enabled>
+	</snapshots>
+	<releases>
+		<enabled>false</enabled>
+	</releases>
+</repository>
 ```
 
 ## Build