Wording about ownership rules; Rc enabling multiple ownership that isn't allowed

[LCamel]

• I have searched open and closed issues and pull requests for duplicates, using these search terms:

  • is:issue There can only be one owner at a time.
  • is:issue "one owner"

• I have checked the latest main branch to see if this has already been fixed, in this file:

  • book/src/ch04-01-what-is-ownership.md

    Line 92 in 71352de

     * There can only be one owner at a time. 

URL to the section(s) of the book with this problem:

• book/src/ch04-01-what-is-ownership.md

  Line 92 in 71352de

   * There can only be one owner at a time. 

• book/src/ch15-04-rc.md

  Line 4 in 71352de

   owns a given value. However, there are cases when a single value might have 

Description of the problem:

In Chapter 4, it says: "Ownership Rules ...... There can only be one owner at a time."
In Chapter 15, it says: "However, there are cases when a single value might have multiple owners."

Is it still a rule?

Suggested fix: