Fix error message readability (#1435)

The multi-line string in the code concatenates imperfectly when actually printed. Need to include spacing manually, unless one gets the header row in the worksheet is not unique.

gspread/worksheet.py

@@ -572,7 +572,7 @@ def get_all_records(
             header_row_is_unique = len(keys) == len(set(keys))
             if not header_row_is_unique:
                 raise GSpreadException(
-                    "the header row in the worksheet is not unique"
+                    "the header row in the worksheet is not unique, "
                     "try passing 'expected_headers' to get_all_records"
                 )
         else: