Fix typo.

Author: randylevy

docs/types/discriminated-unions.md

@@ -109,7 +109,7 @@ function area(s: Shape) {
 
 ### strictNullChecks
 
-If using strictNullChecks and doing exhaustive checks you should return the `_exhaustiveCheck` variable (of type `never`) as well, otherwise TypeScirpt infers a possible return of `undefined`. So:
+If using strictNullChecks and doing exhaustive checks you should return the `_exhaustiveCheck` variable (of type `never`) as well, otherwise TypeScript infers a possible return of `undefined`. So:
 
 ```ts
 function area(s: Shape) {