Fix some scripting tutorial examples

Some of the examples in the scripting tutorial were regularly getting
out of sync with the base scripts (because the line numbering would need
to be updated).  Fixed this maintenance burden by using small example
scripts instead of actual Bro scripts.  These small example scripts
do not need to be kept in sync with the bro base scripts.

doc/scripting/http_main.bro

@@ -0,0 +1,7 @@
+module HTTP;
+
+export {
+	## This setting changes if passwords used in Basic-Auth are captured or
+	## not.
+	const default_capture_password = F &redef;
+}