[Trivial PATCH] Make RST figures true figures

[CasperVector]

As the title. See also image from Text.Pandoc.Builder and para from Text.Pandoc.Readers.Markdown.

diff --git a/src/Text/Pandoc/Readers/RST.hs b/src/Text/Pandoc/Readers/RST.hs
index c12a149..fc52e92 100644
--- a/src/Text/Pandoc/Readers/RST.hs
+++ b/src/Text/Pandoc/Readers/RST.hs
@@ -580,7 +580,7 @@ directive' = do
         "figure" -> do
            (caption, legend) <- parseFromString extractCaption body'
            let src = escapeURI $ trim top
-           return $ B.para (B.image src "" caption) <> legend
+           return $ B.para (B.image src "fig:" caption) <> legend
         "image" -> do
            let src = escapeURI $ trim top
            let alt = B.str $ maybe "image" trim $ lookup "alt" fields