Fix clang warning regarding printf

[tvrusso]

In tube.c, a use of "printf(helpstr)" was flagged by clang (the default compiler on FreeBSD and macOS) 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.