fix some typos

Signed-off-by: cuishuang <imcusg@gmail.com>

user/src/bin/cat.rs

@@ -12,7 +12,7 @@ pub fn main(argc: usize, argv: &[&str]) -> i32 {
     assert!(argc == 2);
     let fd = open(argv[1], OpenFlags::RDONLY);
     if fd == -1 {
-        panic!("Error occured when opening file");
+        panic!("Error occurred when opening file");
     }
     let fd = fd as usize;
     let mut buf = [0u8; 256];