Disable releases on forgerock-snapshots-repository

I've been getting the following exception during my tests:

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-site-plugin:3.3:site (default-site) on project forgerock-doc-maven-plugin-test: SiteToolException: The site descriptor cannot be resolved from the repository: ArtifactResolutionException: Unable to locate site descriptor: Could not transfer artifact org.forgerock:forgerock-parent:xml:site_en:1.2.1 from/to forgerock-snapshots-repository (http://maven.forgerock.org/repo/snapshots): Failed to transfer file: http://maven.forgerock.org/repo/snapshots/org/forgerock/forgerock-parent/1.2.1/forgerock-parent-1.2.1-site_en.xml. Return code is: 409 , ReasonPhrase:Conflict.
[ERROR] org.forgerock:forgerock-parent:xml:1.2.1
[ERROR] 
[ERROR] from the specified remote repositories:
[ERROR] forgerock-staging-repository (http://maven.forgerock.org/repo/releases, releases=true, snapshots=false),
[ERROR] forgerock-snapshots-repository (http://maven.forgerock.org/repo/snapshots, releases=true, snapshots=true),
[ERROR] central (https://repo.maven.apache.org/maven2, releases=true, snapshots=false)
```

This small change makes sure that the snapshots repository isn't contacted for release artifacts