The Cargo.toml in this case is sort of "advisory". When you originally built with 0.8.3 cargo automatically picked 0.8.5 for you, so updating won't change anything.
To see what's really being used, check the automatically generated Cargo.lock file.
(There is a syntax to specify an exact version in Cargo.toml, but this is frowned upon, unless you have very specific reasons to do so).