Update RaycastResult.yaml (#1015)

## Changes

<!-- Please summarize your changes. -->
Changed the references from "instance" to BasePart. Referring to the
type of the intersected object as "instance" is too generic and may be
confusing, since Models and Folders are also instances that some
intersected object could have been a child of, but raycasts can only
return a reference to the object in of itself and never the parent
model. The (poorly named) _instance_ property of RaycastResult can only
ever hold a basepart or a terrain cell, and terrain descends from
basepart so basepart is accurate and more specific than instance.


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reports, etc.). -->

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---------

Author: lack-of-gravity-jack

content/en-us/reference/engine/datatypes/RaycastResult.yaml

@@ -25,7 +25,7 @@ properties:
     tags: []
     deprecation_message: ''
   - name: RaycastResult.Instance
-    type: Instance
+    type: BasePart
     summary: |
       The `Class.BasePart` or `Class.Terrain` cell that the ray intersected.
     description: |
@@ -47,10 +47,10 @@ properties:
   - name: RaycastResult.Position
     type: Vector3
     summary: |
-      The position of the intersection between the ray and the part.
+      The world space point at which the intersection occurred.
     description: |
       The world space point at which the intersection occurred, usually a point
-      directly on the surface of the instance.
+      directly on the surface of the `Class.BasePart` or `Class.Terrain` cell.
     code_samples: []
     tags: []
     deprecation_message: ''