Update pd-t7-3.md (issue #216)

Corrected the structure of the article and the description of the system.

content/3.defense-systems/pd-t7-3.md

@@ -19,7 +19,7 @@ relevantAbstracts:
 # PD-T7-3
 ## Description
 
-PD-T7-3 systems were discovered to be anti-phage systems during an experimental screen performed on *E. coli* strains ref{doi=10.1038/s41564-022-01219-4}
+PD-T7-3 systems were discovered to be anti-phage systems during an experimental screen performed on *E. coli* strains :ref{doi=10.1038/s41564-022-01219-4}
 
 ## Molecular mechanisms
 As far as we are aware, the molecular mechanism of PD-T7-3 is unknown.