docs: i2c: smbus-protocol: fix link syntax

Use the proper ReST syntax to generate a valid hyperlink.
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/i2c/smbus-protocol.rst

@@ -27,8 +27,8 @@ a different protocol operation entirely.
 Each transaction type corresponds to a functionality flag. Before calling a
 transaction function, a device driver should always check (just once) for
 the corresponding functionality flag to ensure that the underlying I2C
-adapter supports the transaction in question. See
-<file:Documentation/i2c/functionality.rst> for the details.
+adapter supports the transaction in question. See :doc:`functionality` for
+the details.
 
 
 Key to symbols