media: ipu3.rst: Improve header formatting on tables

Use the header-rows option of the flat-table directive in order to have
the first row displayed as a header. Also capitalize these headers.
These changes make the tables easier to read.

Signed-off-by: Nícolas F. R. A. Prado <[e-mail]>
Signed-off-by: Sakari Ailus <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/admin-guide/media/ipu3.rst

···
 .. tabularcolumns:: |p{0.8cm}|p{4.0cm}|p{4.0cm}|
 
 .. flat-table::
+    :header-rows: 1
 
-    * - pad
-      - direction
-      - purpose
+    * - Pad
+      - Direction
+      - Purpose
 
     * - 0
       - sink
···
 .. tabularcolumns:: |p{0.8cm}|p{4.0cm}|p{4.0cm}|
 
 .. flat-table::
+    :header-rows: 1
 
-    * - pad
-      - direction
-      - purpose
+    * - Pad
+      - Direction
+      - Purpose
 
     * - 0
       - sink