fix procmail syntax file matching

The default system Vim procmail syntax file match folder without including
backslash and whitespace.
I hope someone can fix it. (I do not know how to match a backslash and a
whitespace into [] matching)

here is the default procmail folder matching. It can not match "Foo\ Bar/Do"
(has "\ ")
syn match procmailActionFolder contained "^\s*[-_a-zA-Z0-9/]\+"

Thanks.

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