docs(customizer): drop a negation the sentence did not need

"tracks every numeric metric its backend reports" already says the list is
not fixed; "not a fixed list" only restated it.

Signed-off-by: Albert Cui <albcui@nvidia.com>

Author: albcui

docs/customizer/tutorials/metrics.mdx

@@ -30,8 +30,8 @@ The time to complete this tutorial is approximately 10 minutes.
 
 ## Available Metrics
 
-A customization job tracks every numeric metric its backend reports, not a fixed
-list. The two you will always find are:
+A customization job tracks every numeric metric its backend reports. The two you
+will always find are:
 
 - **Training Loss** (`train_loss`): recorded every step the training framework logs one
 - **Validation Loss** (`val_loss`): recorded on every validation pass