fix in Nigeria BUR2 KYOTOGHG processing

src/unfccc_ghg_data/unfccc_reader/Nigeria/read_NGA_BUR2_from_pdf.py

@@ -299,7 +299,7 @@ if __name__ == "__main__":
     current_source = data_proc_pm2.coords["source"].to_numpy()[0]
     data_temp = data_proc_pm2.pr.loc[{"source": current_source}]
     data_proc_pm2 = data_proc_pm2.pr.set("source", "BUR_NIR", data_temp)
-    data_proc_pm2 = data_proc_pm2.pr.loc[{"source": "BUR_NIR"}]
+    data_proc_pm2 = data_proc_pm2.pr.loc[{"source": ["BUR_NIR"]}]
 
     # ###
     # save data to IF and native format