validator: Adjust --wait-for-exit message on non-linux (#6708)

The validator exit command can have --monitor OR --wait-for-exit
specified. The functionality for --wait-for-exit is only supported on
linux, and the error message for the non-linux codepath uses the term
"monitor"

Given that the other option is called monitor, this is potentially
confusing. So, adjust the message to use "wait" instead of "monitor"

validator/src/commands/exit/mod.rs

@@ -190,8 +190,8 @@ fn poll_until_pid_terminates(pid: u32) -> Result<()> {
 }
 
 #[cfg(not(target_os = "linux"))]
-fn poll_until_pid_terminates(pid: u32) -> Result<()> {
-    println!("Unable to monitor agave-validator process {pid} on this platform");
+fn poll_until_pid_terminates(_pid: u32) -> Result<()> {
+    println!("Unable to wait for agave-validator process termination on this platform");
     Ok(())
 }