RST writer: fix anchors for headers.

We were missing an `_`.
See #4188.

src/Text/Pandoc/Writers/RST.hs

@@ -250,7 +250,7 @@ blockToRST (Header level (name,classes,_) inlines) = do
           let headerChar = if level > 5 then ' ' else "=-~^'" !! (level - 1)
           let border = text $ replicate (offset contents) headerChar
           let anchor | null name || name == autoId = empty
-                     | otherwise = ".. " <> text name <> ":" $$ blankline
+                     | otherwise = ".. _" <> text name <> ":" $$ blankline
           return $ nowrap $ anchor $$ contents $$ border $$ blankline
     else do
           let rub     = "rubric:: " <> contents

test/command/3937.md

@@ -3,7 +3,7 @@
 # My Great Section {#mysection}
 # Other section
 ^D
-.. mysection:
+.. _mysection:
 
 My Great Section
 ================