multiplot doc spelling errors

[Daniel J S. <dan...@ie...>]

Attached is a patch to fix spelling errors in the mulitplot documentation.

Also, I notice in this documentation that

?nomultiplot

is a means to access this text.  Perhaps all the cases of

?no<keyword>

that appear in gnuplot.doc can be moved under the same heading

?nomultiplot
?noarrow
etc.

Deprecated syntax.  Use `unset <keyword>` instead.