dt-bindings: w1: update w1-gpio.yaml reference

Changeset dd2c898bc2 ("dt-bindings: w1: Convert 1-Wire GPIO binding to a schema")
renamed: Documentation/devicetree/bindings/w1/w1-gpio.txt
to: Documentation/devicetree/bindings/w1/w1-gpio.yaml.

Update its cross-reference accordingly.

Fixes: dd2c898bc2 ("dt-bindings: w1: Convert 1-Wire GPIO binding to a schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/0b415b5e05bcf9d593cb421c96996d914528e732.1631785820.git.mchehab+huawei@kernel.org

Documentation/w1/masters/w1-gpio.rst

@@ -11,7 +11,7 @@ Description
 GPIO 1-wire bus master driver. The driver uses the GPIO API to control the
 wire and the GPIO pin can be specified using GPIO machine descriptor tables.
 It is also possible to define the master using device tree, see
-Documentation/devicetree/bindings/w1/w1-gpio.txt
+Documentation/devicetree/bindings/w1/w1-gpio.yaml
 
 
 Example (mach-at91)