docs: dt: display/ti: fix typos at the devicetree/ directory name

The name of the devicetree directory is wrong on those three
TI bindings:

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/875b824ac97bd76dfe77b6227ff9b6b2671a6abf.1584450500.git.mchehab+huawei@kernel.org

Documentation/devicetree/bindings/display/ti/ti,am65x-dss.yaml

@@ -65,7 +65,7 @@ properties:
   ports:
     type: object
     description:
-      Ports as described in Documentation/devictree/bindings/graph.txt
+      Ports as described in Documentation/devicetree/bindings/graph.txt
     properties:
       "#address-cells":
         const: 1

Documentation/devicetree/bindings/display/ti/ti,j721e-dss.yaml

@@ -98,7 +98,7 @@ properties:
   ports:
     type: object
     description:
-      Ports as described in Documentation/devictree/bindings/graph.txt
+      Ports as described in Documentation/devicetree/bindings/graph.txt
     properties:
       "#address-cells":
         const: 1

Documentation/devicetree/bindings/display/ti/ti,k2g-dss.yaml

@@ -56,7 +56,7 @@ properties:
   port:
     type: object
     description:
-      Port as described in Documentation/devictree/bindings/graph.txt.
+      Port as described in Documentation/devicetree/bindings/graph.txt.
       The DSS DPI output port node
 
   max-memory-bandwidth: