fix broken link to SPI Library reference

[fabianegli]

The URL https://www.arduino.cc/en/reference/SPI was forwarded to https://docs.arduino.cc/language-reference/en/functions/communication/spi/ which returned permission denied.

What This PR Changes

• The link now points directly to the SPI Library reference page.

Contribution Guidelines

• I confirm that I have read the contribution guidelines and comply with them.

[CLAassistant]

All committers have signed the CLA.

[fabianegli]

Why are there two places on the website with the same information?

https://docs.arduino.cc/language-reference/en/functions/communication/SPI/
and
https://reference.arduino.cc/reference/en/language/functions/communication/spi/

Please also note the inconsistent capitalization of "SPI" in the URL. Are there some rules regarding URI normalization?