fix content from directive

sphinx_d2lang/d2lang.py

@@ -16,7 +16,7 @@ class D2langDirective(Directive):
         srcdir = self.state.document.settings.env.srcdir
         diag_source = self.content
         with tempfile.NamedTemporaryFile() as fp:
-            for line in self.content.splitlines():
+            for line in self.content.data:
                 fp.write(bytes(line,'utf-8'))
                 fp.write(bytes('\n','utf-8'))
             fp.seek(0)