fix ref in gaps4.md

content/3.defense-systems/gaps4.md

@@ -18,10 +18,10 @@ relevantAbstracts:
 GAPS4 is a two genes system (GAPS4a and GAPS4b). GAPS stands for GMT-encoded Anti-Phage System. 
 
 GAPS4 is present in both Gram positive and Gram negative 
-GAPS4 activity was assessed in *E. coli* and was shown to be active against T4, P1-vir and lambda-vir and to reduce lysis plaque size of T7 :doi{10.1101/2023.03.28.534373}. 
+GAPS4 activity was assessed in *E. coli* and was shown to be active against T4, P1-vir and lambda-vir and to reduce lysis plaque size of T7 :ref{doi=10.1101/2023.03.28.534373}. 
 
 ## Molecular mechanism
-GAPS4a is a nuclease containing a domain from the PDDEXK clan (CL0236) suggesting that the GAPS4 defense system is acting via DNA degradation. Both genes are required for phage defense and were predicted to form a heterodimer. It was shown that the system works causes host DNA degradation upon infection by lambda-vir only, which would suggest that GAPS4 is an abortive infection system :doi{10.1101/2023.03.28.534373}.
+GAPS4a is a nuclease containing a domain from the PDDEXK clan (CL0236) suggesting that the GAPS4 defense system is acting via DNA degradation. Both genes are required for phage defense and were predicted to form a heterodimer. It was shown that the system works causes host DNA degradation upon infection by lambda-vir only, which would suggest that GAPS4 is an abortive infection system :ref{doi=10.1101/2023.03.28.534373}.
 
 ## Example of genomic structure