clk: shmobile: div6: Grammar s/They/Their/

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>

Documentation/devicetree/bindings/clock/renesas,cpg-div6-clocks.txt

 * Renesas CPG DIV6 Clock
 
 The CPG DIV6 clocks are variable factor clocks provided by the Clock Pulse
-Generator (CPG). They clock input is divided by a configurable factor from 1
+Generator (CPG). Their clock input is divided by a configurable factor from 1
 to 64.
 
 Required Properties: