[FIX] improve longitudinal example (#1576)

* fix longitudinal

* fix

src/longitudinal-and-multi-site-studies.md

@@ -53,7 +53,8 @@ A guide for using macros can be found at
                 "sub-control01_ses-postdrug_phasediff.json": "",
                 "sub-control01_ses-postdrug_magnitude1.nii.gz": "",
                 }
-            }
+            },
+        "sub-control01_sessions.tsv": "",
         },
     "participants.tsv": "",
     "dataset_description.json": "",
@@ -62,6 +63,19 @@ A guide for using macros can be found at
     }
 ) }}
 
+`sub-control01_sessions.tsv` content:
+
+```Text
+session_id	acq_time	systolic_blood_pressure
+ses-predrug	2009-06-15T13:45:30	120
+ses-postdrug	2009-06-16T13:45:30	100
+```
+
+See this [example dataset](https://github.com/bids-standard/bids-examples/tree/master/7t_trt)
+that has been formatted
+using this specification and can be used
+for practical guidance when curating a new longitudinal dataset.
+
 ## Multi-site or multi-center studies
 
 This version of the BIDS specification does not explicitly cover studies with