Update page.mdx to correct "th" to "the" (#12446)

The first paragraph is missing an "e" in its first "the". I added that.

www/apps/book/app/learn/fundamentals/admin/page.mdx

@@ -4,7 +4,7 @@ export const metadata = {
 
 # {metadata.title}
 
-In this chapter, you'll learn about th Medusa Admin dashboard and the possible ways to customize it.
+In this chapter, you'll learn about the Medusa Admin dashboard and the possible ways to customize it.
 
 ## What is the Medusa Admin?