[MPMD-357] - Upgrade to PMD 6.51.0 (#100)

diff --git a/pom.xml b/pom.xml
index e849611..3d8957f 100644
--- a/pom.xml
+++ b/pom.xml
@@ -89,7 +89,7 @@
     <doxiaVersion>1.11.1</doxiaVersion>
     <doxiaSitetoolsVersion>1.11.1</doxiaSitetoolsVersion>
     <javaVersion>8</javaVersion><!-- Because PMD 6.35.0+ requires Java 8 -->
-    <pmdVersion>6.50.0</pmdVersion>
+    <pmdVersion>6.51.0</pmdVersion>
     <slf4jVersion>1.7.36</slf4jVersion>
     <aetherVersion>1.0.0.v20140518</aetherVersion>
     <sitePluginVersion>3.12.1</sitePluginVersion>
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 5d56fda..a61cd09 100644
--- a/src/site/apt/examples/upgrading-PMD-at-runtime.apt.vm
+++ b/src/site/apt/examples/upgrading-PMD-at-runtime.apt.vm
@@ -88,7 +88,7 @@
 *--------------------------------------------------------------------------------*--------------------------------------------------*
 | <<maven-pmd-plugin>>                                                           | <<PMD>>                                          |
 *--------------------------------------------------------------------------------*--------------------------------------------------*
-| {{{https://maven.apache.org/plugins-archives/maven-pmd-plugin-3.20.0/}3.20.0}} | {{{https://pmd.github.io/pmd-6.50.0/}6.50.0}}    |
+| {{{https://maven.apache.org/plugins-archives/maven-pmd-plugin-3.20.0/}3.20.0}} | {{{https://pmd.github.io/pmd-6.51.0/}6.51.0}}    |
 *--------------------------------------------------------------------------------*--------------------------------------------------*
 | {{{https://maven.apache.org/plugins-archives/maven-pmd-plugin-3.19.0/}3.19.0}} | {{{https://pmd.github.io/pmd-6.49.0/}6.49.0}}    |
 *--------------------------------------------------------------------------------*--------------------------------------------------*