Use 2.8-SNAPSHOT in plugin archetype example in plugin documentation

The example uses 2.5-SNAPSHOT which is quite old.

Change it to 2.8-SNAPSHOT to be in line with the version of Gerrit
currently under development.

Change-Id: Ice247fe45799bf884eae7f9181c9f85fdcbcd9ac

Documentation/dev-plugins.txt

diff --git a/Documentation/dev-plugins.txt b/Documentation/dev-plugins.txt
index ad576a0..195a57b 100644
--- a/Documentation/dev-plugins.txt
+++ b/Documentation/dev-plugins.txt
@@ -38,7 +38,7 @@
 ----
 mvn archetype:generate -DarchetypeGroupId=com.google.gerrit \
     -DarchetypeArtifactId=gerrit-plugin-archetype \
-    -DarchetypeVersion=2.5-SNAPSHOT \
+    -DarchetypeVersion=2.8-SNAPSHOT \
     -DgroupId=com.google.gerrit \
     -DartifactId=testPlugin
 ----