media: docs: v4l2-subdev: fix typo

Fix "Helper functions exists" -> "Helper functions exist".
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/driver-api/media/v4l2-subdev.rst

@@ -34,7 +34,7 @@ provides host private data for that purpose that can be accessed with
 From the bridge driver perspective, you load the sub-device module and somehow
 obtain the :c:type:`v4l2_subdev` pointer. For i2c devices this is easy: you call
 ``i2c_get_clientdata()``. For other buses something similar needs to be done.
-Helper functions exists for sub-devices on an I2C bus that do most of this
+Helper functions exist for sub-devices on an I2C bus that do most of this
 tricky work for you.
 
 Each :c:type:`v4l2_subdev` contains function pointers that sub-device drivers