git-filter-repo.txt: fix example of editing blob contents

You can call bytes.replace() or re.sub(), but you can't call
bytes.sub().  Oops.  Fix the example in the documentation.

Reported-by: John Gietzen <john@gietzen.us>
Signed-off-by: Elijah Newren <newren@gmail.com>

Documentation/git-filter-repo.txt

@@ -879,7 +879,7 @@ git filter-repo --blob-callback '
     # Mark this blob for removal from all commits
     blob.skip()
   else:
-    blob.data = blob.data.sub(b"Hello", b"Goodbye")
+    blob.data = blob.data.replace(b"Hello", b"Goodbye")
   '
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