title: Schema is not configured for mutations
body: <p>I have the following schema : </p>
  
  <pre><code>import {
    GraphQLSchema,
    GraphQLObjectType,
    GraphQLInt,
    GraphQLString
  } from 'graphql';
  let counter = 100;
  const schema = new GraphQLSchema({
   // Browse: http://localhost:3000/graphql?query={counter,message}
    query: new GraphQLObjectType({
      name: 'Query',
      fields: () =&gt; ({
        counter: {
          type: GraphQLInt,
          resolve: () =&gt; counter
        },
        message: {
          type: GraphQLString,
          resolve: () =&gt; 'Salem'
        }
      })
    }),
    mutiation: new GraphQLObjectType({
      name: 'Mutation',
      fields: () =&gt; ({
        incrementCounter: {
          type: GraphQLInt,
          resolve: () =&gt; ++counter
        }
      })
    })
  })
  export default schema;
  </code></pre>
  
  <p>The following query works fine: </p>
  
  <pre><code>{counter, message}
  </code></pre>
  
  <p>However, <code>mutation {incrementCounter}</code> throws the following errors : </p>
  
  <pre><code>{
    "data": null,
    "errors": [
      {
        "message": "Schema is not configured for mutations",
        "locations": [
          {
            "line": 1,
            "column": 1
          }
        ]
      }
    ]
  }
  </code></pre>
  
  <p>Known that the server is : </p>
  
  <pre><code>import GraphQLHTTP from 'express-graphql';
  const app = express();
  app.use('/graphql',GraphQLHTTP({schema}));
  </code></pre>
  
  <p>What's the missing thing that makes mutation configured ?</p>