Correct options label and documentation for pdu head/tail

[jganser]

Correct options label and documentation

Tail- was actually corresponding to tail+ in Unix Head+ as it was Head- was wrongly named in options label, though correctly named in the documentation.

Looked it up in the cc file, where naming was correct and matching the UNIX behavior.

As can also be seen by my bash tryouts.

echo "123456789" | tail -c 3
89

echo "123456789" | tail -c -3
89

echo "123456789" | tail -c +3
3456789

echo "123456789" | head -c 3
123⏎

echo "123456789" | head -c -3
1234567⏎ 

echo "123456789" | head -c +3
123⏎ 

[jganser]

Found the mismatch in the gr310 build on Ubuntu gr-satellites version 5.3.0-2

[daniestevez]

Thanks! Indeed reviewing the C++ code I find that these were correctly named in there, so I don't know why I goofed up when writing the GRC file.