Fix const examples (#7610)

# Description

Fix const examples

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

crates/nu-command/src/core_commands/const_.rs

@@ -72,12 +72,12 @@ impl Command for Const {
         vec![
             Example {
                 description: "Create a new parse-time constant.",
-                example: "let x = 10",
+                example: "const x = 10",
                 result: None,
             },
             Example {
                 description: "Create a composite constant value",
-                example: "let x = { a: 10, b: 20 }",
+                example: "const x = { a: 10, b: 20 }",
                 result: None,
             },
         ]