usage-model.txt: fix typo machine_init->init_machine

machine_init was used intead of init_machine which clearly was not
intended.
Signed-off-by: NRichard Genoud <richard.genoud@gmail.com>
Signed-off-by: NRob Herring <rob.herring@calxeda.com>

Documentation/devicetree/usage-model.txt

@@ -312,7 +312,7 @@ device tree for the NVIDIA Tegra board.
 	};
 };
 
-At .machine_init() time, Tegra board support code will need to look at
+At .init_machine() time, Tegra board support code will need to look at
 this DT and decide which nodes to create platform_devices for.
 However, looking at the tree, it is not immediately obvious what kind
 of device each node represents, or even if a node represents a device