docs: fix link to x-nullable docs from README (#1135)

README.md

@@ -213,7 +213,7 @@ ReDoc makes use of the following [vendor extensions](https://swagger.io/specific
 * [`x-traitTag`](docs/redoc-vendor-extensions.md#x-traitTag) - useful for handling out common things like Pagination, Rate-Limits, etc
 * [`x-code-samples`](docs/redoc-vendor-extensions.md#x-code-samples) - specify operation code samples
 * [`x-examples`](docs/redoc-vendor-extensions.md#x-examples) - specify JSON example for requests
-* [`x-nullable`](docs/redoc-vendor-extensions.md#nullable) - mark schema param as a nullable
+* [`x-nullable`](docs/redoc-vendor-extensions.md#x-nullable) - mark schema param as a nullable
 * [`x-displayName`](docs/redoc-vendor-extensions.md#x-displayname) - specify human-friendly names for the menu categories
 * [`x-tagGroups`](docs/redoc-vendor-extensions.md#x-tagGroups) - group tags by categories in the side menu
 * [`x-servers`](docs/redoc-vendor-extensions.md#x-servers) - ability to specify different servers for API (backported from OpenAPI 3.0)