doc: update the update command (#153)

it looks like with the release of 4.0, AstroUpdatePackages was removed and AstroUpdate does the same thing (it calls astrocore and updates everything)

Author: ALameLlama

src/content/docs/index.mdx

@@ -168,7 +168,7 @@ docker run -w /root -it --rm alpine:edge sh -uelic '
 
 - **Update Mason packages and plugins**
 
-  Run `:AstroUpdatePackages` (`<Leader>pa`) to update both Neovim plugins and Mason packages
+  Run `:AstroUpdate` (`<Leader>pa`) to update both Neovim plugins and Mason packages
 
 - **Check AstroNvim version**