Fix swapped reference numbering in get_bsrn docstring (#2680)

Author: aman-coder03

pvlib/iotools/bsrn.py

@@ -148,9 +148,9 @@ def get_bsrn(station, start, end, username, password,
         <https://bsrn.awi.de/>`_
     .. [2] `BSRN Data Retrieval via FTP
        <https://bsrn.awi.de/data/data-retrieval-via-ftp/>`_
-    .. [4] `BSRN Data Release Guidelines
+    .. [3] `BSRN Data Release Guidelines
        <https://bsrn.awi.de/data/conditions-of-data-release/>`_
-    .. [3] `Update of the Technical Plan for BSRN Data Management, 2013,
+    .. [4] `Update of the Technical Plan for BSRN Data Management, 2013,
        Global Climate Observing System (GCOS) GCOS-174.
        <https://bsrn.awi.de/fileadmin/user_upload/bsrn.awi.de/Publications/gcos-174.pdf>`_
     """  # noqa: E501