docs: update ref to discriminated-unions docs (#2485)

Co-authored-by: Ryan Vermooten <ryan.vermooten@vdl.digital>

Author: rvermootenct

ERROR_HANDLING.md

@@ -24,7 +24,7 @@ Each ZodError has an `issues` property that is an array of `ZodIssues`. Each iss
 
 ## ZodIssue
 
-`ZodIssue` is _not_ a class. It is a [discriminated union](https://www.typescriptlang.org/docs/handbook/advanced-types.html#discriminated-unions).
+`ZodIssue` is _not_ a class. It is a [discriminated union](https://www.typescriptlang.org/docs/handbook/2/narrowing.html#discriminated-unions).
 
 The link above is the best way to learn about the concept. Discriminated unions are an ideal way to represent a data structures that may be one of many possible variants. You can see all the possible variants defined [here](./src/ZodError.ts). They are also described in the table below if you prefer.