Update creating-basic-heads.md mapping instructions (#849)

## Changes

Update creating-basic-heads.md mapping instructions to include a
specific example of the correct convention for custom property names
that map frame number to FACS pose.

The specific "Frame#" format is only visible in the videos and gave me
trouble the other day while trying to make poses with zero prior
experience.

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Co-authored-by: haan-roblox <[email protected]>

Author: CAZ-H

content/en-us/art/characters/facial-animation/creating-basic-heads.md

@@ -419,7 +419,7 @@ To map your saved poses and the RootFaceJoint:
 
 5. Click the **Type** dropdown, then select **String**.
 6. In the **Property Name** field:
-   1. If you are mapping a pose, input the **frame number** you are mapping.
+   1. If you are mapping a pose, input the **frame number** you are mapping, **Frame0** for example.
    2. If you are mapping the RootFaceJoint, input **RootFaceJoint**.
 7. Leave the **Default Value** and **Description** fields empty.
 8. Click the **OK** button. The new custom property updates with your new property name.