Remove SmallVector mention

SmallVector is long gone, as it's been first replaced
by OneVector in commit e5e6375352,
which then has been removed entirely in favour of SmallVec in
commit 130a32fa72.

compiler/rustc_data_structures/src/thin_vec.rs

@@ -2,7 +2,7 @@ use crate::stable_hasher::{HashStable, StableHasher};
 
 use std::iter::FromIterator;
 
-/// A vector type optimized for cases where this size is usually 0 (cf. `SmallVector`).
+/// A vector type optimized for cases where this size is usually 0 (cf. `SmallVec`).
 /// The `Option<Box<..>>` wrapping allows us to represent a zero sized vector with `None`,
 /// which uses only a single (null) pointer.
 #[derive(Clone, Encodable, Decodable, Debug)]