In programmatic usage docs, "each plugin should be an object" needs clarification

```
import { buildSchema } from 'graphql';
import * as fs from 'fs';
import * as typescriptPlugin from '@graphql-codegen/typescript';
import { printSchema, parse, GraphQLSchema } from 'graphql';

const schema: GraphQLSchema = buildSchema(`type A {}`);
const outputFile = 'relative/pathTo/filename.ts';
const config = {
// used by a plugin internally, although the 'typescript' plugin currently
// returns the string output, rather than writing to a file
filename: outputFile,
schema: parse(printSchema(schema)),
plugins: [ // Each plugin should be an object
{
typescript: {}, // Here you can pass configuration to the plugin
},
],
pluginMap: {
typescript: typescriptPlugin,
},
};
```

I did this and it took some time to realize what I was doing wrong:

```
plugins: [ // Each plugin should be an object
{
typescript: {}, // Here you can pass configuration to the plugin
foo: {},
},
],
```

An example with two plugins should be in the docs.