Fix indentation

It should be 4 spaces, not 2

schemars/src/lib.rs

@@ -134,6 +134,7 @@ fn main() {
             "type": "boolean"
         },
         "myNullableEnum": {
+            "default": null,
             "anyOf": [
                 {
                     "$ref": "#/definitions/MyEnum"
@@ -164,7 +165,6 @@ fn main() {
 ```
 </details>
 
-\
 `#[serde(...)]` attributes can be overriden using `#[schemars(...)]` attributes, which behave identically (e.g. `#[schemars(rename_all = "camelCase")]`). You may find this useful if you want to change the generated schema without affecting Serde's behaviour, or if you're just not using Serde.
 
 ## Feature Flags