rust-mode.el: Tweak syntax table

"_" should keep the default syntax class (symbol, not word). This
allows, e.g., `forward-word' to behave in the familiar way, jumping to
underscores within a function or variable name.
auto merge of #19021 : roysc/rust/emacs-pr, r=brson

"_" should keep the default syntax class (symbol, not word). This
allows, e.g., `forward-word' to behave in the familiar way, jumping to
underscores within a function or variable name.