doc-rst: extended-controls: "count" is a constant

Constify the string "count" where it means a field.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
diff --git a/Documentation/linux_tv/media/v4l/extended-controls.rst b/Documentation/linux_tv/media/v4l/extended-controls.rst
index 50598b4..413f6ca 100644
--- a/Documentation/linux_tv/media/v4l/extended-controls.rst
+++ b/Documentation/linux_tv/media/v4l/extended-controls.rst
@@ -61,7 +61,7 @@
 All controls in the control array must belong to the specified control
 class. An error is returned if this is not the case.
 
-It is also possible to use an empty control array (count == 0) to check
+It is also possible to use an empty control array (``count`` == 0) to check
 whether the specified control class is supported.
 
 The control array is a struct