Add AllUsers scope example to Update section per review feedback

The update/reinstall section only showed the default CurrentUser scope,
inconsistent with Check and Uninstall sections that show both scopes.
Added -Scope AllUsers variant with elevation note to prevent users from
accidentally creating a separate per-user install when updating a
system-wide font.

Author: MariusStorhaug

README.md

@@ -81,6 +81,12 @@ NerdFont is outdated. To ensure you have the latest version, reinstall the font
 Install-NerdFont -Name 'FiraCode' -Force
 ```
 
+If the font was originally installed for all users, update it with the matching scope (requires elevated privileges):
+
+```powershell
+Install-NerdFont -Name 'FiraCode' -Force -Scope AllUsers
+```
+
 This downloads and installs the font from the latest NerdFonts release, overwriting any existing version.
 
 ### Uninstall a NerdFont