Minor fix in Node class documentation

The name of the property responsible for physics fps is fixed:
this is `Engine.iterations_per_second`, not `Engine.target_fps`.

(cherry picked from commit 6e5cd36b12b535f31a2b453c39723a77adc182ed)

doc/classes/Node.xml

@@ -326,7 +326,7 @@
 			<return type="float">
 			</return>
 			<description>
-				Returns the time elapsed since the last physics-bound frame (see [method _physics_process]). This is always a constant value in physics processing unless the frames per second is changed via [member Engine.target_fps].
+				Returns the time elapsed since the last physics-bound frame (see [method _physics_process]). This is always a constant value in physics processing unless the frames per second is changed via [member Engine.iterations_per_second].
 			</description>
 		</method>
 		<method name="get_position_in_parent" qualifiers="const">
@@ -745,7 +745,7 @@
 			<argument index="0" name="enable" type="bool">
 			</argument>
 			<description>
-				Enables or disables physics (i.e. fixed framerate) processing. When a node is being processed, it will receive a [constant NOTIFICATION_PHYSICS_PROCESS] at a fixed (usually 60 FPS, see [member Engine.target_fps] to change) interval (and the [method _physics_process] callback will be called if exists). Enabled automatically if [method _physics_process] is overridden. Any calls to this before [method _ready] will be ignored.
+				Enables or disables physics (i.e. fixed framerate) processing. When a node is being processed, it will receive a [constant NOTIFICATION_PHYSICS_PROCESS] at a fixed (usually 60 FPS, see [member Engine.iterations_per_second] to change) interval (and the [method _physics_process] callback will be called if exists). Enabled automatically if [method _physics_process] is overridden. Any calls to this before [method _ready] will be ignored.
 			</description>
 		</method>
 		<method name="set_physics_process_internal">