dt-bindings: i2c: i2c-mux: spelling s/required is/required if/

Fix both instances.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NPeter Rosin <peda@axentia.se>

Documentation/devicetree/bindings/i2c/i2c-mux.txt

@@ -6,10 +6,10 @@ multiplexer/switch will have one child node for each child bus.
 
 Optional properties:
 - #address-cells = <1>;
-   This property is required is the i2c-mux child node does not exist.
+   This property is required if the i2c-mux child node does not exist.
 
 - #size-cells = <0>;
-   This property is required is the i2c-mux child node does not exist.
+   This property is required if the i2c-mux child node does not exist.
 
 - i2c-mux
    For i2c multiplexers/switches that have child nodes that are a mixture