Fix confusing example in paste(1)

Paste's man page contains an example for a reimplementation of
nl(1). This example uses the command line

sed = myfile | paste -s -d '\t\n' - -

in order to concatenate consecutive lines with an intervening tab.

However, the way the example uses the switches -s and -d and two dash
input files is redundant. There are in fact two equivalent but simpler
ways to achieve the desired result:

sed = myfile | paste -s -d '\t\n' -

uses the same style as the previous example, while

sed = myfile | paste - -

is arguably even simpler and illustrates the final sentence of the
DESCRIPTION.

Reviewed by: imp@
Pull Request: https://github.com/freebsd/freebsd-src/pull/163