fix(nextjs): Use lower camel case for webpack plugin options object (#4621)

It's not a class or an interface, so no reason it should be capitalized.

Author: lobsterkatie

src/platforms/javascript/guides/nextjs/manual-setup.mdx

@@ -163,7 +163,7 @@ const moduleExports = {
   // your existing module.exports
 };
 
-const SentryWebpackPluginOptions = {
+const sentryWebpackPluginOptions = {
   // Additional config options for the Sentry Webpack plugin. Keep in mind that
   // the following options are set automatically, and overriding them is not
   // recommended:
@@ -177,7 +177,7 @@ const SentryWebpackPluginOptions = {
 
 // Make sure adding Sentry options is the last code to run before exporting, to
 // ensure that your source maps include changes from all other Webpack plugins
-module.exports = withSentryConfig(moduleExports, SentryWebpackPluginOptions);
+module.exports = withSentryConfig(moduleExports, sentryWebpackPluginOptions);
 ```
 
 <Alert level="warning" title="Serverless Environments">
@@ -192,7 +192,7 @@ Make sure to add the Sentry config last; otherwise, the source maps the plugin r
 
 By default, `withSentryConfig` will add an instance of `SentryWebpackPlugin` to the webpack plugins, for both server and client builds. This means that when you run a production build (`next build`), `sentry-cli` will automatically detect and upload your source files, source maps, and bundles to Sentry, so that your stacktraces can be demangled. (This behavior is disabled when running the dev server (`next dev`), because otherwise the full upload process would reoccur on each file change.)
 
-To configure the plugin, pass a `SentryWebpackPluginOptions` argument to `withSentryConfig`, as seen in the example above. All available options are documented [here](https://github.com/getsentry/sentry-webpack-plugin#options).
+To configure the plugin, pass a `sentryWebpackPluginOptions` argument to `withSentryConfig`, as seen in the example above. All available options are documented [here](https://github.com/getsentry/sentry-webpack-plugin#options).
 
 If you want or need to handle source map uploading separately, the plugin can be disabled for either the server or client build process. To do this, add a `sentry` object to `moduleExports` above, and set the relevant options there:
 
@@ -205,7 +205,7 @@ const moduleExports = {
 };
 ```
 
-If you disable the plugin for both server and client builds, it's safe to omit the `SentryWebpackPluginOptions` parameter from your `withSentryConfig` call:
+If you disable the plugin for both server and client builds, it's safe to omit the `sentryWebpackPluginOptions` parameter from your `withSentryConfig` call:
 
 ```javascript {filename:next.config.js}
 module.exports = withSentryConfig(moduleExports);