Minor: Examples cleanup + more docs in pruning example (apache#11086)

* Fix ticks

* Update comments

datafusion-examples/README.md

@@ -62,10 +62,10 @@ cargo run --example csv_sql
 - [`function_factory.rs`](examples/function_factory.rs): Register `CREATE FUNCTION` handler to implement SQL macros
 - [`make_date.rs`](examples/make_date.rs): Examples of using the make_date function
 - [`memtable.rs`](examples/memtable.rs): Create an query data in memory using SQL and `RecordBatch`es
-- ['parquet_index.rs'](examples/parquet_index.rs): Create an secondary index over several parquet files and use it to speed up queries
+- [`parquet_index.rs`](examples/parquet_index.rs): Create an secondary index over several parquet files and use it to speed up queries
 - [`parquet_sql.rs`](examples/parquet_sql.rs): Build and run a query plan from a SQL statement against a local Parquet file
 - [`parquet_sql_multiple_files.rs`](examples/parquet_sql_multiple_files.rs): Build and run a query plan from a SQL statement against multiple local Parquet files
-- ['parquet_exec_visitor.rs'](examples/parquet_exec_visitor.rs): Extract statistics by visiting an ExecutionPlan after execution
+- [`parquet_exec_visitor.rs`](examples/parquet_exec_visitor.rs): Extract statistics by visiting an ExecutionPlan after execution
 - [`parse_sql_expr.rs`](examples/parse_sql_expr.rs): Parse SQL text into Datafusion `Expr`.
 - [`plan_to_sql.rs`](examples/plan_to_sql.rs): Generate SQL from Datafusion `Expr` and `LogicalPlan`
 - [`pruning.rs`](examples/parquet_sql.rs): Use pruning to rule out files based on statistics

datafusion-examples/examples/pruning.rs

@@ -33,6 +33,11 @@ use std::sync::Arc;
 /// quickly eliminate entire files / partitions / row groups of data from
 /// consideration using statistical information from a catalog or other
 /// metadata.
+///
+/// This example uses a user defined catalog to supply pruning information, as
+/// one might do as part of a higher level storage engine. See
+/// `parquet_index.rs` for an example that uses pruning in the context of an
+/// individual query.
 #[tokio::main]
 async fn main() {
     // In this example, we'll use the PruningPredicate to determine if