docs: Fix typo in getting-started/installation-and-setup.md (#4306)

Summary:
Fix a typo in the relay config file example in the documentation,
should be `excludes` instead of `exclude`.

Pull Request resolved: #4306

Reviewed By: tyao1

Differential Revision:
D45711796

Privacy Context Container: L1125407

Pulled By: captbaritone

fbshipit-source-id: 50b082f9e80f7b14f3e0bc59fe404626fb9b7265

website/docs/getting-started/installation-and-setup.md

@@ -69,7 +69,7 @@ module.exports = {
   src: "./src",
   language: "javascript", // "javascript" | "typescript" | "flow"
   schema: "./data/schema.graphql",
-  exclude: ["**/node_modules/**", "**/__mocks__/**", "**/__generated__/**"],
+  excludes: ["**/node_modules/**", "**/__mocks__/**", "**/__generated__/**"],
 }
 ```