Fixed typo in webpack.config.dev.js (#3601)

Replaced `it's` with `its`

packages/react-scripts/config/webpack.config.dev.js

@@ -220,7 +220,7 @@ module.exports = {
           // that fall through the other loaders.
           {
             // Exclude `js` files to keep "css" loader working as it injects
-            // it's runtime that would otherwise processed through "file" loader.
+            // its runtime that would otherwise processed through "file" loader.
             // Also exclude `html` and `json` extensions so they get processed
             // by webpacks internal loaders.
             exclude: [/\.(js|jsx|mjs)$/, /\.html$/, /\.json$/],