#15 Ground conductors documented incorrectly

[Bob Techentin]

Section 6 of the Users' Guide says that you can specify
a ground wire by prefixing the conductor name with "G"
or "g," but the real magic incantation is "gr." The
documentation should be fixed.

[Bob Techentin]

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Changed the user guide to correctly state that the ground
line must be prefixed with "gr" in lower case.