Corrigendum to "Spectroscopic and structural properties of CeO2 nanocrystals doped with La3+, Nd3+ and modified on their surface with Ag nanoparticles" [Heliyon 7 (5) (May 2021) e06958]. Issue 6 (June 2021) - Michalska, Monika; Lemański, Karol; Sikora, Andrzej