Fix clang warning regarding printf

In tube.c, a use of "printf(helpstr)" was flagged by clang as
potentially insecure.  The insecurity of this is probably
insignificant, but fixing it removes a warning from compilation.

The warning is:
   src/tube.c:303:24: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
  303 |                 printf(helpStr);
      |                        ^~~~~~~
src/tube.c:303:24: note: treat the string as an argument to avoid this
  303 |                 printf(helpStr);
      |                        ^

All that need be done is to provide a format string that is a string
literal, with helpStr being the string to print instead of the format
string.

src/tube.c

@@ -300,7 +300,7 @@ void tube_init(int argc, char* argv[])
         }
 
         if ((strcmp(argv[firstArg],"-h") == 0) || (strcmp(argv[firstArg],"--help") == 0)){
-                printf(helpStr);
+                printf("%s",helpStr);
                 exit(0);
         }