Remove unnecessary echo uses from examples (#7500)

`echo` tends to confuse new Nu users; they expect it to work like
`print` when it just passes a value to the next stage of the pipeline.

We haven't quite figured out what to do about `echo` in the long run,
but I think a good start is to remove `echo` from command examples where
it would be unnecessary and arguably unidiomatic.

crates/nu-command/src/math/variance.rs

@@ -45,7 +45,7 @@ impl Command for SubCommand {
         vec![
             Example {
                 description: "Get the variance of a list of numbers",
-                example: "echo [1 2 3 4 5] | math variance",
+                example: "[1 2 3 4 5] | math variance",
                 result: Some(Value::float(2.0, Span::test_data())),
             },
             Example {