docs: update tutorial with new diff headers
I forgot to update the tutorial in the previous commit.
@@ -89,7 +89,7 @@ to say that you want an already added file to not be tracked
To see the diff, run `jj diff`:
```shell script
$ jj diff
-modified file README.md:
+Modified regular file at README.md:
...
4 4: ## Disclaimer
5 5:
@@ -400,7 +400,7 @@ the "D" line. Then close Meld. If we look the diff of the second commit, we
now see that all three lines got capitalized:
```shell script
$ jj diff -r ::@
-modified file file:
+Modified regular file at file:
1 1: aA
2 2: bB
3 3: cC
