Fixed another brush typo.md (#930)

Fixed it from saying "Set Base Size to 3" to saying "Set Brush Size to
3"

## Changes

<!-- Please summarize your changes. -->
In Roblox Studio the terrain editor says brush size. The guide is saying
to change the base size. I fixed the typo to say "Set Brush Size to 3"
<!-- Please link to any applicable information (forum posts, bug
reports, etc.). -->

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Author: BizkitKing

content/en-us/tutorials/curriculums/core/building/create-an-environment-with-terrain.md

@@ -161,7 +161,7 @@ To apply materials to the island:
    </video>
 
 1. Navigate back to the **Terrain Editor** window, then in the **Brush Settings** and **Material Settings** sections,
-   1. Set **Base Size** to **3**.
+   1. Set **Brush Size** to **3**.
    1. Set **Material** to **Grass**.
 1. In the viewport, drag along the edges of the island to apply blades of grass, leaving space in the middle of the island for
    the spawn location and the initial platforms.