docs: Fix typo in nullability operators section
@@ -114,7 +114,7 @@ of `BoxableExpression` as it may change the resulting sql type there. As a possi
there we recommend to use one of the following functions:
* `NullableExpressionMethods::nullable()`
-* `NullableExpressionMethods::assume_not_nullable()`
+* `NullableExpressionMethods::assume_not_null()`
## Changed nullability of array elements<a name="#2-0-0-nullability-of-array-elements"></a>
