dt-bindings: wakeup-source: update reference to m8921-keypad.yaml

commit 53ed3233 ("dt-bindings: input: qcom,pm8921-keypad: convert to
YAML format") resulted in a renaming of the output .txt file from
qcom,pm8xxx-keypad.txt to qcom,pm8921-keypad.yaml.

This patch makes a corresponding update to the link to that .txt file
in wakeup-source.txt.

Flagged by make htmldocs:
Warning: Documentation/devicetree/bindings/power/wakeup-source.txt references a file that doesn't exist: Documentation/devicetree/bindings/input/qcom,pm8xxx-keypad.txt
Signed-off-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240908-keypad-wakeup-ref-v1-1-762e4641468a@kernel.orgSigned-off-by: Rob Herring (Arm) <robh@kernel.org>

Documentation/devicetree/bindings/power/wakeup-source.txt

@@ -26,7 +26,7 @@ List of legacy properties and respective binding document
 3. "linux,wakeup"		Documentation/devicetree/bindings/input/gpio-matrix-keypad.txt
 				Documentation/devicetree/bindings/mfd/tc3589x.txt
 				Documentation/devicetree/bindings/input/touchscreen/ads7846.txt
-4. "linux,keypad-wakeup"	Documentation/devicetree/bindings/input/qcom,pm8xxx-keypad.txt
+4. "linux,keypad-wakeup"	Documentation/devicetree/bindings/input/qcom,pm8921-keypad.yaml
 5. "linux,input-wakeup"		Documentation/devicetree/bindings/input/samsung,s3c6410-keypad.yaml
 6. "nvidia,wakeup-source"	Documentation/devicetree/bindings/input/nvidia,tegra20-kbc.txt