pingserver: fix minor spelling error in help message

refs #2845

Change-Id: I2d1e8d015cb0797dce515e1ed8c700d479ad5ed5
diff --git a/tools/ping/server/ndn-ping-server.cpp b/tools/ping/server/ndn-ping-server.cpp
index a642ed1..0a02805 100644
--- a/tools/ping/server/ndn-ping-server.cpp
+++ b/tools/ping/server/ndn-ping-server.cpp
@@ -41,8 +41,8 @@
 {
   std::cout << "Usage: ndnpingserver [options] ndn:/name/prefix\n"
       "\n"
-      "Starts a NDN ping server that responds to Interests with name"
-      "ndn:/name/prefix/ping/number.\n"
+      "Starts a NDN ping server that responds to Interests under name "
+      "ndn:/name/prefix/ping.\n"
       "\n";
   std::cout << options;
   exit(2);