docs: fix typo in expr.Reference doc (#1343)

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## What changes are included in this PR?

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This PR is to fix typos for 2 functions in the `expr.Reference` doc:

- `is_in()`
- `is_not_in()`

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Yes.

Author: burmecia

crates/iceberg/src/expr/term.rs

@@ -257,7 +257,7 @@ impl Reference {
         Predicate::Unary(UnaryExpression::new(PredicateOperator::NotNull, self))
     }
 
-    /// Creates an is-in expression. For example, `a IS IN (5, 6)`.
+    /// Creates an is-in expression. For example, `a IN (5, 6)`.
     ///
     /// # Example
     ///
@@ -278,7 +278,7 @@ impl Reference {
         ))
     }
 
-    /// Creates an is-not-in expression. For example, `a IS NOT IN (5, 6)`.
+    /// Creates an is-not-in expression. For example, `a NOT IN (5, 6)`.
     ///
     /// # Example
     ///