rust: error: fix the description for `ECHILD`

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: 266def2a0f5b ("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>