(doc) Fix sample in Upgrading PMD at runtime

Thanks to https://github.com/boris-github for reporting
Refs pmd/pmd#402
diff --git a/src/site/apt/examples/upgrading-PMD-at-runtime.apt.vm b/src/site/apt/examples/upgrading-PMD-at-runtime.apt.vm
index 367d9c1..5223c0f 100644
--- a/src/site/apt/examples/upgrading-PMD-at-runtime.apt.vm
+++ b/src/site/apt/examples/upgrading-PMD-at-runtime.apt.vm
@@ -38,7 +38,7 @@
 +-----+
 <project>
   <properties>
-    <pmdVersion>...choose your version...</version>
+    <pmdVersion>...choose your version...</pmdVersion>
   </properties>
 ...
   <build>