checkstyle.xml: simplify a diagnostic message

Author: stephengold

config/checkstyle/checkstyle.xml

@@ -287,7 +287,7 @@
         </module>
         <module name="RegexpSinglelineJava">
             <property name="format" value="\{[ ]+\}"/>
-            <property name="message" value="Empty blocks should have no spaces. Empty blocks may only be represented as '{}' when not part of a multi-block statement (4.1.3)"/>
+            <property name="message" value="Empty blocks must not contain spaces."/>
         </module>
 
         <!-- Checks for size violations:             -->