fix(gatsby-plugin-typescript): typo in readme (#17640)

remove extra . cause error in npm install.

packages/gatsby-plugin-typescript/README.md

@@ -21,7 +21,7 @@ module.exports = {
 }
 ```
 
-_**Please note**: If packages don't ship with TypeScript definitions you'll need to manually install those type definitions, e.g. for React. A typical Gatsby project would need: `npm install --save-dev @types/react @types/react-dom @types/node.`_
+_**Please note**: If packages don't ship with TypeScript definitions you'll need to manually install those type definitions, e.g. for React. A typical Gatsby project would need: `npm install --save-dev @types/react @types/react-dom @types/node`_
 
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