title: Gatsby GraphQL custom date formatString
body: <p>I have this:</p>
  
  <pre><code>export const pageQuery = graphql`
    query {
      site {
        siteMetadata {
          title
        }
      }
      allMdx(sort: { fields: [frontmatter___date], order: DESC }) {
        edges {
          node {
            excerpt
            fields {
              slug
            }
            frontmatter {
              date(formatString: "DD 'de'  MMMM, YYYY", locale: "pt")
              title
              description
            }
          }
        }
      }
    }
  `
  </code></pre>
  
  <p>in the line ->> 
  <code>
  date(formatString: "DD 'de'  MMMM, YYYY", locale: "pt")
  </code>
  I have to insert strings but this 'de' isnt working. I know I want to display the date like:
  <code>
  25 de March, 2020.
  </code>
  But the result is:
  <code>
  25 '32' March, 2020.
  </code>
  I know this isnt working, i know why, but I can't make it right.
  I'm Using Gatsbyjs with graphql</p>