Fixed plain text Servlet class name.

jetty-servlet/source_code

@@ -1,4 +1,4 @@
 ./jetty-servlet/src/main/java/hello/
 ./jetty-servlet/src/main/java/hello/HelloWebServer.java
 ./jetty-servlet/src/main/java/hello/JsonServlet.java
-./jetty-servlet/src/main/java/hello/FreetextServlet.java
+./jetty-servlet/src/main/java/hello/PlaintextServlet.java