fix some typos

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

os/src/drivers/block/sdcard.rs

@@ -423,7 +423,7 @@ impl</*'a,*/ X: SPI> SDCard</*'a,*/ X> {
             /* Byte 15 */
             CSD_CRC: (csd_tab[15] & 0xFE) >> 1,
             Reserved4: 1,
-            /* Return the reponse */
+            /* Return the response */
         })
     }
 

os/src/drivers/chardev/ns16550a.rs

@@ -12,7 +12,7 @@ use volatile::{ReadOnly, Volatile, WriteOnly};
 bitflags! {
     /// InterruptEnableRegister
     pub struct IER: u8 {
-        const RX_AVALIABLE = 1 << 0;
+        const RX_AVAILABLE = 1 << 0;
         const TX_EMPTY = 1 << 1;
     }
 
@@ -95,7 +95,7 @@ impl NS16550aRaw {
         mcr |= MCR::REQUEST_TO_SEND;
         mcr |= MCR::AUX_OUTPUT2;
         read_end.mcr.write(mcr);
-        let ier = IER::RX_AVALIABLE;
+        let ier = IER::RX_AVAILABLE;
         read_end.ier.write(ier);
     }
 

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];