fix(gatsby-plugin-sitemap): allow paths with trailing slash in exclude option

[notzenox]

Now gatsby-plugin-sitemap treats /foo/bar/ and /foo/bar as the same,
which removes unnecessary confusion when using the exclude option,
as previously gatsby-plugin-sitemap wouldn't work correctly with trailing slashes in exclude option.

Also fixed some inconsistencies in README.md

[laurieontech]

Looks good from my end! Will wait for @gatsbyjs/core to merge.

[vladar]

Trailing slashes often cause weird issues but let's hope we won't see any 🤞 (at least I don't see any at the moment). Let's get this merged, thanks for the PR!

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