Rollup merge of #33743 - royalstream:royalstream-stack-doc, r=steveklabnik

Book: small improvement to a table to make it clearer

This table is used as an example of four heap values where two of them got deallocated leaving a gap.
It also has stack variables.
Instead of four stack variables I propose three, that way there's no misleading connection between the number of stack variables and heap variables.
But more importantly: three of the four stack variables had the same name (**y**) which could be confusing to a beginner, I changed this as well.