Fix typos

src/timer.h

@@ -6,7 +6,7 @@ struct mg_timer {
 #define MG_TIMER_REPEAT 1   // Call function periodically, otherwise run once
 #define MG_TIMER_RUN_NOW 2  // Call immediately when timer is set
   void (*fn)(void *);       // Function to call
-  void *arg;                // Function agrument
+  void *arg;                // Function argument
   unsigned long expire;     // Expiration timestamp in milliseconds
   struct mg_timer *next;    // Linkage in g_timers list
 };