docs(graphql): Fix incorrect import

The existing example will error with the following message:

```
❯ npm start

> [email protected] start
> nest start

src/common/plugins/complexity.plugin.ts:1:29 - error TS2305: Module '"@nestjs/graphql"' has no exported member 'Plugin'.

1 import { GraphQLSchemaHost, Plugin } from "@nestjs/graphql";
                              ~~~~~~

Found 1 error(s).
```

This same change was made in the samples a few months ago: nestjs/nest@d8ba5d1#diff-5b158b02130ed8dc4bbebe1b6b42e0ecee7573c44e3850dd013c26765dd52b67

Author: m-allanson

content/graphql/complexity.md

@@ -19,7 +19,8 @@ $ npm install --save graphql-query-complexity
 Once the installation process is complete, we can define the `ComplexityPlugin` class:
 
 ```typescript
-import { GraphQLSchemaHost, Plugin } from '@nestjs/graphql';
+import { GraphQLSchemaHost } from "@nestjs/graphql";
+import { Plugin } from "@nestjs/apollo";
 import {
   ApolloServerPlugin,
   GraphQLRequestListener,