docs: update tutorial with removed "at" from diff headers
I forgot to include this in the previous commit.
@@ -81,7 +81,7 @@ 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 regular file at README.md:
+Modified regular file README.md:
...
4 4: ## Disclaimer
5 5:
@@ -391,7 +391,7 @@ 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 regular file at file:
+Modified regular file file:
1 1: aA
2 2: bB
3 3: cC
