nanorst: add missing blank to exception message

src/borg/nanorst.py

@@ -105,8 +105,8 @@ def rst_to_text(text, state_hook=None, references=None):
                     try:
                         out.write(references[ref])
                     except KeyError:
-                        raise ValueError("Undefined reference in Archiver help: %r — please add reference substitution"
-                                         "to 'rst_plain_text_references'" % ref)
+                        raise ValueError("Undefined reference in Archiver help: %r — please add reference "
+                                         "substitution to 'rst_plain_text_references'" % ref)
                     continue
                 if char == ':' and text.peek(2) == ':\n':  # End of line code block
                     text.read(2)