chart: fix docs to make examples at least buildable

Without importing orphan instances error messages
hardly make sense for newcomer:

> chart.hs:9:5:
>     No instance for (Graphics.Rendering.Chart.Simple.Internal.IsPlot
>                        t0)
>       arising from a use of `plotPS'
>     The type variable `t0' is ambiguous
>     Possible fix: add a type signature that fixes these type variable(s)
>     Note: there are several potential instances:
>       instance (Graphics.Rendering.Chart.Simple.Internal.IsPlot p,
>                 Graphics.Rendering.Chart.Simple.Internal.IsPlot q) =>
>                Graphics.Rendering.Chart.Simple.Internal.IsPlot (p, q)
>         -- Defined in `Graphics.Rendering.Chart.Simple.Internal'
>       instance (Graphics.Rendering.Chart.Simple.Internal.IsPlot p,
>                 Graphics.Rendering.Chart.Simple.Internal.IsPlot q,
>                 Graphics.Rendering.Chart.Simple.Internal.IsPlot r) =>
>                Graphics.Rendering.Chart.Simple.Internal.IsPlot (p, q, r)
>         -- Defined in `Graphics.Rendering.Chart.Simple.Internal'
>       instance (Real a, Real b, Fractional a, Fractional b) =>
>                Graphics.Rendering.Chart.Simple.Internal.IsPlot (a -> b)
>         -- Defined in `Graphics.Rendering.Chart.Simple.Internal'
>       ...plus four others
>     In a stmt of a 'do' block:
>       plotPS "foo.ps" [0, 0.1 .. 10] (sin . exp) "- " (sin . exp) "o-"
>     In the expression:
>       do { plotPNG "foo.png" ds sin "- " cos ". " cos "o";
>            plotPS "foo.ps" [0, 0.1 .. 10] (sin . exp) "- " (sin . exp) "o-" }
>     In an equation for `main':
>         main
>           = do { plotPNG "foo.png" ds sin "- " cos ". " cos "o";
>                  plotPS "foo.ps" [0, 0.1 .. 10] (sin . exp) "- " (sin . exp) "o-" }

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>