docs: Documentation corrections

Installation instructions incorrectly suggested to install ndn-cxx
package from Ubuntu PPA archive.  Instead, ndn-cxx-dev package should be
installed.

Change-Id: I68a645011f31ba7b3a63fda0fb2811006a83e8ef
diff --git a/docs/source/getting-started.rst b/docs/source/getting-started.rst
index a56fbe1..7d3e927 100644
--- a/docs/source/getting-started.rst
+++ b/docs/source/getting-started.rst
@@ -142,7 +142,11 @@
 
        .. code-block:: bash
 
-           sudo apt-get install ndn-cxx
+           sudo apt-get install ndn-cxx-dev
+
+       If you are using ndn-cxx from PPA, please make sure that you do not have the existing installation
+       of ndn-cxx library compiled from source (e.g., in /usr/local).  Otherwise, the version mistmach
+       may result in compilation errors.
 
 - Compile NS-3 with ndnSIM module