docs: i2c: instantiating-devices: fix internal hyperlink

Use ReST syntax so that a proper hyperlink is generated.
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/instantiating-devices.rst

@@ -86,7 +86,7 @@ Method 1c: Declare the I2C devices via ACPI
 -------------------------------------------
 
 ACPI can also describe I2C devices. There is special documentation for this
-which is currently located at Documentation/firmware-guide/acpi/enumeration.rst.
+which is currently located at :doc:`../firmware-guide/acpi/enumeration`.
 
 
 Method 2: Instantiate the devices explicitly