A reminder that a block is a single expr in closures

This is something that's only been briefly mentioned in the beginning of
the tutorial and all of the closure examples within this subsection
include only one expression between { and }.
auto merge of #7003 : alco/rust/tutorial-block-expr, r=bstrie

This is something that's only been briefly mentioned in the beginning of
the tutorial and all of the closure examples within this subsection
include only one expression between { and }.