manpage: Use -H instead of adding /dev/null to searched files

[Mateusz Okulus]

grep manpage example uses an additional /dev/null file to make grep
print out the name of the file that matches in case '*g*.h' glob matches
only one file. I believe the -H option was added exactly for this use
case, that is to make grep always print file names, even if only one was
given. Because of that I think the example should be changed to use the -H
flag to something like grep -Hn -- 'f.*\.c$' *g*.h. The description
should also be changed appropriately.

I haven't filed any copyright assignments so unfortunately I'll abstain
from providing a patch, but this is only a small documentation change.

Regards,
mmokulus

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