Use -F instead of -f in tail(1) examples

There is an example in tail(1) manual page explaining how to use tail(1) to
track the contents of /var/log/messages. The example uses the -f flag to
follow the file. The problem with the -f flag is that it cannot handle the
situation where /var/log/messages is rotated. Hence, use -F instead in the
example.

Reviewed by:	asomers
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D24157

usr.bin/tail/tail.1

@@ -31,7 +31,7 @@
 .\"	@(#)tail.1	8.1 (Berkeley) 6/6/93
 .\" $FreeBSD$
 .\"
-.Dd April 10, 2018
+.Dd March 22, 2020
 .Dt TAIL 1
 .Os
 .Sh NAME
@@ -157,7 +157,7 @@ Keep
 .Pa /var/log/messages
 open, displaying to the standard output anything appended to the file:
 .Pp
-.Dl $ tail -f /var/log/messages
+.Dl $ tail -F /var/log/messages
 .Sh SEE ALSO
 .Xr cat 1 ,
 .Xr head 1 ,