Fix a typo [skip ci]

build.xml

@@ -98,7 +98,7 @@
 	</target>
 
 	<target name="run" description="Runs a main class from the &quot;example&quot; folder.">
-		<fail message="You must specify an example to run using -D:class=&lt;example&gt;." unless="class"/>
+		<fail message="You must specify an example to run using -Dclass=&lt;example&gt;." unless="class"/>
 		<java sourceFile="example/${class}.java" classPathRef="project.classpath" failOnError="true" fork="true"/>
 	</target>