clocksource/drivers/arm_global_timer: Fix Kconfig "its" grammar

Use the possessive "its" instead of the contraction "it's"
where appropriate.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lore.kernel.org/r/20220715015852.12523-1-rdunlap@infradead.orgSigned-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/Kconfig

@@ -367,7 +367,7 @@ config ARM_GT_INITIAL_PRESCALER_VAL
 	depends on ARM_GLOBAL_TIMER
 	help
 	  When the ARM global timer initializes, its current rate is declared
-	  to the kernel and maintained forever. Should it's parent clock
+	  to the kernel and maintained forever. Should its parent clock
 	  change, the driver tries to fix the timer's internal prescaler.
 	  On some machs (i.e. Zynq) the initial prescaler value thus poses
 	  bounds about how much the parent clock is allowed to decrease or