of: Grammar s/property exist/property exists/

Signed-off-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

include/linux/of.h

···
  * @propname:	name of the property to be searched.
  *
  * Search for a property in a device node.
- * Returns true if the property exist false otherwise.
+ * Returns true if the property exists false otherwise.
  */
 static inline bool of_property_read_bool(const struct device_node *np,
 					 const char *propname)