rust: error: fix the description for ECHILD

A mistake was made and the description of `ECHILD` is wrong (it reuses
the description of `ENOEXEC`). This fixes it to reflect what's in
`errno-base.h`.

Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Finn Behrens <me@kloenk.dev>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Fixes: 266def2 ("rust: error: add codes from `errno-base.h`")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230930144958.46051-1-wedsonaf@gmail.com
[ Use the plural, as noticed by Benno. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: wedsonaf

rust/kernel/error.rs

@@ -37,7 +37,7 @@ pub mod code {
     declare_err!(E2BIG, "Argument list too long.");
     declare_err!(ENOEXEC, "Exec format error.");
     declare_err!(EBADF, "Bad file number.");
-    declare_err!(ECHILD, "Exec format error.");
+    declare_err!(ECHILD, "No child processes.");
     declare_err!(EAGAIN, "Try again.");
     declare_err!(ENOMEM, "Out of memory.");
     declare_err!(EACCES, "Permission denied.");