Submodules: update `git submodule update --remote` behaviour

[phil-blain]

• [x] I provide my work under the project license.
• [X] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

Since f0a96e8d4c (submodule: fall back to remote's HEAD for missing remote..branch, 2020-06-24), which went into Git 2.28, 'git submodule upadte --remote' does not default to using the 'master' branch of the remote repository, but rather the remote repository's HEAD branch.

Tweak the wording to reflect this behaviour change.

Context

Noticed while re-reading the chapter today.

[phil-blain]

@ben any comments on this one ?:)