Fix spelling (#2069)

Changed "Conditinal" to "Conditional"

Author: m-sekhon

typescript.md

@@ -244,7 +244,7 @@ type Point = { x: number; y: number };
 type P = keyof Point; // x | y
 ```
 
-## Conditinal Types
+## Conditional Types
 
 ```ts
 // SomeType extends OtherType ? TrueType : FalseType;