typo fix (#48)

Author: guanqun

src/cmd/mod.rs

@@ -46,7 +46,7 @@ impl Command {
         let mut parse = Parse::new(frame)?;
 
         // All redis commands begin with the command name as a string. The name
-        // is read and converted to lower casae in order to do case sensitive
+        // is read and converted to lower cases in order to do case sensitive
         // matching.
         let command_name = parse.next_string()?.to_lowercase();
 

src/server.rs

@@ -225,7 +225,7 @@ impl Listener {
             //
             // `acquire` returns a permit that is bound via a lifetime to the
             // semaphore. When the permit value is dropped, it is automatically
-            // returned to the sempahore. This is convenient in many cases.
+            // returned to the semaphore. This is convenient in many cases.
             // However, in this case, the permit must be returned in a different
             // task than it is acquired in (the handler task). To do this, we
             // "forget" the permit, which drops the permit value **without**
@@ -253,7 +253,7 @@ impl Listener {
                 // connection, a permit is added back to the semaphore.
                 limit_connections: self.limit_connections.clone(),
 
-                // Receive shutdown notifcations.
+                // Receive shutdown notifications.
                 shutdown: Shutdown::new(self.notify_shutdown.subscribe()),
 
                 // Notifies the receiver half once all clones are