`JJ-INSTRUCTIONS` for `split` are wrong

[Vampire]

Steps to Reproduce the Problem

1. jj split

Expected Behavior

Should probably be:

Adjust the right side until it shows the contents you want to keep in the original commit.
The changes that are not selected will be split into the new child commit.

Actual Behavior

Adjust the right side until it shows the contents you want to split into the
new commit.
The changes that are not selected will replace the original commit.

Specifications

• Platform: all
• Version: 0.40.0