Fix math e usage text (#7406)

Closes #7401

# Tests + Formatting

Doesn't apply, doc fix

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

@@ -17,7 +17,7 @@ impl Command for SubCommand {
  }
 
  fn usage(&self) -> &str {
- "Returns the mathematical constant π."
+ "Returns the mathematical constant e (exp(1)/'1 | math exp')."
  }
 
  fn search_terms(&self) -> Vec<&str> {