docs(main.txt): mark preface as such

scop · scop · commit 0602158c1f0c · 2021-12-29T22:58:13.000+02:00
Prevents asciidoctor error for it about an invalid part, without a
section.
diff --git a/doc/main.txt b/doc/main.txt
@@ -3,6 +3,7 @@
 Freddy Vulto (FVu)
 v1.0, Mar 2009
 
+[preface]
 = Preface
 
 Bash completion extends bashs standard completion behavior to achieve
