Corrected documentation to be more gramatically correct (facebook#35141)

Summary:
The documentation for `useColorScheme()` suggested that the user's preferred color scheme will be 'Dark Mode'. Instead suggesting 'Dark Mode' as an example of what the user's preferred color scheme could be is more correct.

## Changelog
[INTERNAL] [FIXED] - Edited documentation for `useColorScheme()` hook
Edited
```javascript
/**
 * A new useColorScheme hook is provided as the preferred way of accessing
 * the user's preferred color scheme (aka Dark Mode).
 */
export function useColorScheme(): ColorSchemeName;
```
to
```javascript
/**
 * A new useColorScheme hook is provided as the preferred way of accessing
 * the user's preferred color scheme (e.g. Dark Mode).
 */
export function useColorScheme(): ColorSchemeName;
```

Pull Request resolved: facebook#35141

Test Plan: Documentation only - no testing required.

Reviewed By: cipolleschi

Differential Revision: D40934781

Pulled By: rshest

fbshipit-source-id: acac8947c3f99016839be27f505066e8992a20fa

Author: willsawyerrrr

Libraries/Utilities/Appearance.d.ts

@@ -38,6 +38,6 @@ export namespace Appearance {
 
 /**
  * A new useColorScheme hook is provided as the preferred way of accessing
- * the user's preferred color scheme (aka Dark Mode).
+ * the user's preferred color scheme (e.g. Dark Mode).
  */
 export function useColorScheme(): ColorSchemeName;