Suggest "sudo lsof" rather than just "lsof"

When invoking px on a single PID, suggest using "sudo lsof" rather than
just "lsof" to get the full list of file descriptors for a process.

I sometimes copy / paste these suggestions, and a lot of times when I do
that I have to add "sudo" manually to get it working.

README.rst

@@ -130,7 +130,7 @@ This shows detailed info about PID 80727.
     mDNSResponder(201)
       [unix] ->0xe32cbd7be6021f1f
 
-  For a list of all open files, do "lsof -p 80727", or "watch lsof -p 80727" for a live view.
+  For a list of all open files, do "sudo lsof -p 80727", or "sudo watch lsof -p 80727" for a live view.
 
 * The command line has been split with one argument per line. This makes long
   command lines readable.

px/px_processinfo.py

@@ -174,8 +174,8 @@ def print_fds(process, processes):
             print("    " + channel_name)
 
     print("")
-    print("For a list of all open files, do \"lsof -p {0}\", "
-          "or \"watch lsof -p {0}\" for a live view.".format(process.pid))
+    print("For a list of all open files, do \"sudo lsof -p {0}\", "
+          "or \"sudo watch lsof -p {0}\" for a live view.".format(process.pid))
 
     if os.getuid() != 0:
         print("")