Update maximum width of reference HTML

This change upgrade the framework to consume the newly-published
docbook-reference-plugin version 0.2.1 containing a maximum width
for the HTML reference guide.

build.gradle

@@ -3,7 +3,7 @@ buildscript {
         maven { url 'http://repo.springsource.org/plugins-release' }
     }
     dependencies {
-        classpath 'org.springframework.build.gradle:docbook-reference-plugin:0.2.0'
+        classpath 'org.springframework.build.gradle:docbook-reference-plugin:0.2.1'
     }
 }