Valid categories in toml descriptions are out of date

[carols10cents]

ThreadSafety/thread-safety is a valid category https://docs.rs/rustsec/latest/rustsec/advisory/enum.Category.html

But thread-safety doesn't appear in

advisory-db/README.md

Lines 78 to 80 in f414b4d

	# Optional: Categories this advisory falls under. Valid categories are:
	# "code-execution", "crypto-failure", "denial-of-service", "file-disclosure"
	# "format-injection", "memory-corruption", "memory-exposure", "privilege-escalation"

or

advisory-db/EXAMPLE_ADVISORY.md

Lines 7 to 8 in f414b4d

	# Valid categories: "code-execution", "crypto-failure", "denial-of-service", "file-disclosure"
	# "format-injection", "memory-corruption", "memory-exposure", "privilege-escalation"

[tarcieri]

We should probably just give a couple few example categories, but then link to the rustdoc rather than trying to explicitly maintain a list of categories here that could go out of date in the future if more categories are added.

That would also help inform people of the category descriptions.