Only remove -spec/-type from first line

Before this change a spec looking like this:

```
-spec foo(integer()) -> ok;
         (atom()) -> ok.
```

would be transformed to:

```
-spec foo(integer()) -> ok;
    (atom()) -> ok.
```

that is the second clause would be indented incorrectly.

Author: garazdawi

lib/ex_doc/language/erlang.ex

@@ -728,10 +728,6 @@ defmodule ExDoc.Language.Erlang do
   defp trim_offset(binary, offset) do
     binary
     |> String.trim()
-    |> String.split("\n")
-    |> Enum.map(fn line ->
-      binary_part(line, offset, byte_size(line) - offset)
-    end)
-    |> Enum.join("\n")
+    |> then(&binary_part(&1, offset, byte_size(&1) - offset))
   end
 end