DOC: fix read_bsrn reference

[amandadiniz-eng]

• Closes Correct referencing in pvlib.iotools.read_bsrn #2684
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Description

Corrects the reference in the read_bsrn docstring from [3] (BSRN Data Retrieval via FTP) to [4] (BSRN Data Release Guidelines).

Testing

Tests were not run because this is a documentation-only reference correction.

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