Typo fixes on Markdown files (#31380)

Some simple typo fixes on Markdown files

docs/api-reference/data-fetching/getInitialProps.md

@@ -139,6 +139,6 @@ For more information on what to do next, we recommend the following sections:
 <div class="card">
   <a href="/docs/advanced-features/automatic-static-optimization.md">
     <b>Automatic Static Optimization:</b>
-    <small>Learn about how Nextjs automatically optimizes your pages.</small>
+    <small>Learn about how Next.js automatically optimizes your pages.</small>
   </a>
 </div>

docs/api-reference/next.config.js/headers.md

@@ -365,7 +365,7 @@ module.exports = {
         headers: [
           {
             key: 'x-hello',
-            value: 'worlld',
+            value: 'world',
           },
         ],
       },

docs/api-reference/next/image.md

@@ -123,7 +123,7 @@ If you are using `layout="fill"` or `layout="responsive"`, it's important to ass
 
 For example, when the parent element will constrain the image to always be less than half the viewport width, use `sizes="50vw"`. Without `sizes`, the image will be sent at twice the necessary resolution, decreasing performance.
 
-If you are using `layout="intrinsic"` or `layout="fixed"`, then `sizes` is not needed because the upperbound width is constrained already.
+If you are using `layout="intrinsic"` or `layout="fixed"`, then `sizes` is not needed because the upper bound width is constrained already.
 
 [Learn more](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/img#attr-sizes).
 

docs/testing.md

@@ -114,7 +114,7 @@ Since Cypress is testing a real Next.js application, it requires the Next.js ser
 
 Run `npm run build` and `npm run start`, then run `npm run cypress` in another terminal window to start Cypress.
 
-> **Note:** Alternatively, you can install the `start-server-and-test` package and add it to the `package.json` scripts field: `"test": "start-server-and-test start http://localhost:3000 cypress"` to start the Next.js production server in conjuction with Cypress. Remember to rebuild your application after new changes.
+> **Note:** Alternatively, you can install the `start-server-and-test` package and add it to the `package.json` scripts field: `"test": "start-server-and-test start http://localhost:3000 cypress"` to start the Next.js production server in conjunction with Cypress. Remember to rebuild your application after new changes.
 
 ### Getting ready for Continuous Integration (CI)