Fix typo in JavaScript roadmap (kamranahmedse#5369)

Correcting the typo.

src/data/roadmaps/typescript/content/101-typescript-types/104-undefined.md

@@ -1,6 +1,6 @@
 # undefined
 
-JavaScript has two primitive values used to signal absent or uninitialized value: `null` (absent) and `undefined` (unintialized).
+JavaScript has two primitive values used to signal absent or uninitialized value: `null` (absent) and `undefined` (uninitialized).
 
 TypeScript has two corresponding _types_ by the same names. How these types behave depends on whether you have the `strictNullChecks` option on.