docs(configuration): fix heading levels for performance

Author: snitin315

src/content/configuration/performance.mdx

@@ -18,7 +18,7 @@ This feature was inspired by the idea of [webpack Performance Budgets](https://g
 
 Configure how performance hints are shown. For example if you have an asset that is over 250kb, webpack will emit a warning notifying you of this.
 
-## `performance.assetFilter`
+### performance.assetFilter
 
 `function(assetFilename) => boolean`
 
@@ -45,7 +45,7 @@ module.exports = {
 
 The example above will only give you performance hints based on `.js` files.
 
-## `performance.hints`
+### performance.hints
 
 `string = 'warning': 'error' | 'warning'` `boolean: false`
 
@@ -86,7 +86,7 @@ module.exports = {
 
 An error will be displayed notifying you of a large asset. We recommend using `hints: "error"` during production builds to help prevent deploying production bundles that are too large, impacting webpage performance.
 
-## `performance.maxAssetSize`
+### performance.maxAssetSize
 
 `number = 250000`
 
@@ -101,7 +101,7 @@ module.exports = {
 };
 ```
 
-## `performance.maxEntrypointSize`
+### performance.maxEntrypointSize
 
 `number = 250000`