fix typo

app.py

@@ -15,7 +15,7 @@ def compute_horvath_score(uploaded_file, clock_selection, coefficients, intercep
     
     # the user data must have the required columns
     if 'probes' not in user_data.columns or 'beta' not in user_data.columns:
-        return "Error: Uploaded CSV must have columns 'cpg_sites' and 'beta'."
+        return "Error: Uploaded CSV must have columns 'probes' and 'beta'."
     
     user_data.rename(columns={"probes": "Marker"}, inplace=True)