Use the correct syscall name in panic message

I copied it from the getrandom code but forgot to change the name.
Reported by Sebastien Marie.

src/libstd/rand/os.rs

@@ -233,7 +233,7 @@ fn fill_bytes(&mut self, v: &mut [u8]) {
             for s in v.chunks_mut(256) {
                 unsafe { ret = syscall(7, s.as_mut_ptr(), s.len()); }
                 if ret == -1 {
-                    panic!("unexpected getrandom error: {}", errno());
+                    panic!("unexpected getentropy error: {}", errno());
                 }
             }
         }