chore(docs): Clean up "Adding a Path Prefix" doc (#16048)
chore(docs): Clean up "Adding a Path Prefix" doc (#16048)

fix typo: programatic -> programmatic

[Use “you” as the pronoun:](https://www.gatsbyjs.org/contributing/gatsby-style-guide/#use-you-as-the-pronoun) we -> you

Plus general grammar and readability improvements :mag:
gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby