Fix running examples readme

Some examples are runnable from any place (e.g. `csv_sql`), but some
expect a specific working directory (e.g. `regexp`). Running from
`datafusion-examples/examples` is tested on CI so guaranteed to work,
let's put this path in the README.

As a follow-up, we should look what it would take to make examples
runnable directly from an IDE such as RustRover.

datafusion-examples/README.md

@@ -35,6 +35,9 @@ cd datafusion
 # Download test data
 git submodule update --init
 
+# Change to the examples directory
+cd datafusion-examples/examples
+
 # Run the `csv_sql` example:
 # ... use the equivalent for other examples
 cargo run --example csv_sql