docs: Clarify sample term required for Mean storage (#536)

* Use the same information as provided by boost-histogram in scikit-hep/boost-histogram#876
scikit-hep · Sep 13, 2023 · a9602fc · a9602fc
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commit a9602fc
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diff --git a/docs/user-guide/storages.rst b/docs/user-guide/storages.rst
@@ -128,7 +128,7 @@ Mean
 
 This storage tracks a "Profile", that is, the mean value of the accumulation instead of the sum.
 It stores the count (as a double), the mean, and a term that is used to compute the variance. When
-filling, you can add a ``sample=`` term.
+filling, you must add a ``sample=`` term.
 
 
 WeightedMean