Fix Peter's affiliation (#1754)

9b4eb73 introduced wrong affiliation.

Author: maszyman

_gsocproposals/2025/proposal_ATLAS_lossy_compression.md

@@ -18,7 +18,7 @@ project_mentors:
   - email: [email protected]
     first_name: Peter
     last_name: Van Gemmeren
-    organization: CERN
+    organization: ANL
 ---
 
 ## Description