Fix comment typo

My bad :(

libc/include/stdlib.h

@@ -63,13 +63,13 @@ extern "C"
     /* Register a handler to be run at normal program termination. */
     int atexit(void (*func)(void));
 
-    /* Parse a decimal integer from the a string. */
+    /* Parse a decimal integer from a string. */
     int atoi(const char* s);
 
-    /* Parse a decimal integer from the a string. */
+    /* Parse a decimal integer from a string. */
     long atol(const char* s);
 
-    /* Parse a decimal integer from the a string. */
+    /* Parse a decimal integer from a string. */
     long long atoll(const char* s);
 
     double atof(const char*);