Fix typo

Author: spastorino

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -75,7 +75,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericP
 
     const NO_GENERICS: &hir::Generics<'_> = hir::Generics::empty();
 
-    // We use an `IndexSet` to preserves order of insertion.
+    // We use an `IndexSet` to preserve order of insertion.
     // Preserving the order of insertion is important here so as not to break UI tests.
     let mut predicates: FxIndexSet<(ty::Predicate<'_>, Span)> = FxIndexSet::default();