fix incorrect config schema in Docs (`TypeSpecProjectSchema.output-dir`) (#4965)

## description

The schema that was incorrectly referred to as `boolean` in the
documentation has been corrected to the correct one.

## referense

- relevant code

78140e410d/website/src/content/docs/docs/handbook/configuration/configuration.md?plain=1#L32

- schema source code

78140e410d/packages/compiler/src/config/config-schema.ts (L46-L49)

website/src/content/docs/docs/handbook/configuration/configuration.md

@@ -29,7 +29,7 @@ model TypeSpecProjectSchema {
   parameters?: Record<{default: string}>
   "environment-variables"?: Record<{default: string}>
   "warn-as-error"?: boolean;
-  "output-dir"?: boolean;
+  "output-dir"?: string;
   "trace"?: string | string[];
   imports?: string;
   emit?: string[];