Use correct error message casing for io::const_errors

Error messages are supposed to start with lowercase letters, but a lot
of `io::const_error` messages did not. This fixes them to start with a
lowercase letter.
I did consider adding a const check for this to the macro, but some of
them start with proper nouns that make sense to uppercase them.

See https://doc.rust-lang.org/1.85.0/std/error/trait.Error.html

library/std/src/sys/pal/unix/process/process_unix.rs

@@ -1228,7 +1228,7 @@ mod linux_child_ext {
                 .as_ref()
                 // SAFETY: The os type is a transparent wrapper, therefore we can transmute references
                 .map(|fd| unsafe { mem::transmute::<&imp::PidFd, &os::PidFd>(fd) })
-                .ok_or_else(|| io::const_error!(ErrorKind::Uncategorized, "No pidfd was created."))
+                .ok_or_else(|| io::const_error!(ErrorKind::Uncategorized, "no pidfd was created."))
         }
 
         fn into_pidfd(mut self) -> Result<os::PidFd, Self> {