RST writer: fix backslash escaping after strings

The check whether a complex inline element following a string must be
escaped, now depends on the last character of the string instead of the
first.

Fixes: #5906

test/Tests/Writers/RST.hs

@@ -105,6 +105,9 @@ tests = [ testGroup "rubrics"
           , "keeps quotes" =:
             strong (str "f" <> doubleQuoted (str "d") <> str "l") =?>
             "**f“d”l**"
+          , "backslash inserted between str and code" =:
+            str "/api?query=" <> code "foo" =?>
+            "/api?query=\\ ``foo``"
           ]
         , testGroup "headings"
           [ "normal heading" =: