sed.1: Adjust examples to fit in one line + SPDX

[ziaee]

Summary

Examples were previously overflowing at MANWIDTH 80. Keep the same
complexity but use shorter phrases to make them easier to read. Tag
SPDX while here.

MFC after: 3 days

[emaste]

usr.bin/sed/sed.1

608

etc might lead to some confusion as there is a /etc, but also /home/example to /usr/local/example seems like an odd transformation.

maybe s#/bin/bash#/bin/sh# as a short example that could be an actually used example?

[ziaee]

Thanks @emaste! If we could get it one character shorter it would even
work at MANWIDTH=59. I revised the other example as well for this effect

[ziaee]

So, since this shouldn't be controversial, if there are no complaints, I'll commit this in a week?

[bcr]

Good to go!