Fix invalid variable name in input command docs (#6716)

crates/nu-command/src/platform/input.rs

@@ -149,7 +149,7 @@ impl Command for Input {
     fn examples(&self) -> Vec<Example> {
         vec![Example {
             description: "Get input from the user, and assign to a variable",
-            example: "let user-input = (input)",
+            example: "let user_input = (input)",
             result: None,
         }]
     }