Update nnpdf_data/nnpdf_data/commondata/ATLAS_PH_13TEV_139fb/metadata.yaml

Co-authored-by: Juan M. Cruz-Martinez <juacrumar@lairen.eu>

Author: vschutze-alt

nnpdf_data/nnpdf_data/commondata/ATLAS_PH_13TEV_139fb/metadata.yaml

@@ -41,7 +41,7 @@ implemented_observables:
     variables:
       eta:
         description: "Absolute photon pseudorapidity"
-        label: "$|\eta|$"
+        label: r"$|\eta|$"
         units: ""
       ET:
         description: "Transverse energy"