fix: update out-of-date doc about datafusion (#1183)

# Description

Fix out-of-date doc about querying datafusion.

# Related Issue(s)
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- closes #106
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# Documentation

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rust/src/lib.rs

@@ -52,10 +52,10 @@
 //! Querying from local filesystem:
 //! ```ignore
 //! use std::sync::Arc;
-//! use datafusion::execution::context::ExecutionContext;
+//! use datafusion::prelude::SessionContext;
 //!
 //! async {
-//!   let mut ctx = ExecutionContext::new();
+//!   let mut ctx = SessionContext::new();
 //!   let table = deltalake::open_table("./tests/data/simple_table")
 //!       .await
 //!       .unwrap();