docs(gatsby-source-wordpress): Update README.md to make example code work

[thien-hoang]

createPage() in the example codes needs the slug returned from graphql.

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[sidharthachatterjee]

Good catch! Thank you, @thien-hoang 👍

[gatsbot]

Holy buckets, @thien-hoang — we just merged your PR to Gatsby! 💪💜

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