docs/tutorial: remove reference to jj amend

`jj amend` is discouraged and no longer a visible alias of `jj squash` since jj-vcs#3252. This
is the only remaining occurrence of it in the docs. We should recommend using `jj
squash` directly instead.

docs/tutorial.md

@@ -153,8 +153,7 @@ If we later realize that we want to make further changes, we can make them in
 the working copy and then run `jj squash`. That command squashes (moves) the
 changes from a given commit into its parent commit. Like most commands, it acts
 on the working-copy commit by default. When run on the working-copy commit, it
-behaves very similar to `git commit --amend`, and `jj amend` is in fact an alias
-for `jj squash`.
+behaves very similar to `git commit --amend`.
 
 Alternatively, we can use `jj edit <commit>` to resume editing a commit in the
 working copy. Any further changes in the working copy will then amend the