RST writer: don't treat 'example' as a syntax name.

This fixes conversions from org with example blocks.
Closes #4748.

src/Text/Pandoc/Writers/RST.hs

@@ -273,7 +273,8 @@ blockToRST (CodeBlock (_,classes,kvs) str) = do
      then return $ prefixed "> " (text str) $$ blankline
      else return $
           (case [c | c <- classes,
-                     c `notElem` ["sourceCode","literate","numberLines"]] of
+                     c `notElem` ["sourceCode","literate","numberLines",
+                                  "number-lines","example"]] of
              []       -> "::"
              (lang:_) -> (".. code:: " <> text lang) $$ numberlines)
           $+$ nest 3 (text str) $$ blankline

test/command/4748.md

@@ -0,0 +1,16 @@
+```
+% pandoc -f org -t rst
+Before example block.
+#+begin_example
+This is in an example block.
+#+end_example
+After example block.
+^D
+Before example block.
+
+::
+
+   This is in an example block.
+
+After example block.
+```