all: fix codespell errors

Reviewed-by: Jay Satiro
Reviewed-by: Daniel Stenberg
Closes https://github.com/curl/curl/pull/5452

tests/sshserver.pl

@@ -1120,7 +1120,7 @@ if ($sshdid =~ /OpenSSH-Windows/) {
     # Put an "exec" in front of the command so that the child process
     # keeps this child's process ID by being tied to the spawned shell.
     exec("exec $cmd") || die "Can't exec() $cmd: $!";
-    # exec() will create a new process, but ties the existance of the
+    # exec() will create a new process, but ties the existence of the
     # new process to the parent waiting perl.exe and sh.exe processes.
 
     # exec() should never return back here to this process. We protect