formatting json example

README.md

@@ -85,6 +85,7 @@ a corpus of pre-annotated text for other tasks (e.g. figure caption to graphic,
 ## Dataset Structure
 
 ### Data Instances
+```json
 {
     "accession_id": "PMC8515580",
     "pmid": "34661004",
@@ -459,6 +460,7 @@ a corpus of pre-annotated text for other tasks (e.g. figure caption to graphic,
     "citation": "ACS Omega. 2021 Sep 29; 6(40):26489-26498",
     "package_file": "oa_package/ca/94/PMC8515580.tar.gz"
 }
+```json
 
 ### Data Fields