lpd(8): document correct control character for DVI

[Lennart Jablonka]

r is for FORTRAN output, not for DVI.

Index: lpd.8
===================================================================
RCS file: /cvs/src/usr.sbin/lpr/lpd/lpd.8,v
retrieving revision 1.32
diff -u -p -r1.32 lpd.8
--- lpd.8       10 Oct 2022 09:13:43 -0000      1.32
+++ lpd.8       6 Jun 2023 12:30:54 -0000
@@ -242,7 +242,7 @@ The file contains troff output (cat phot
 .It n
 Ditroff File.
 The file contains device independent troff output.
-.It r
+.It d
 DVI File.
 The file contains
 .Tn Tex l