Correct more typos (#500)

libc/stdio/appendr.c

@@ -41,7 +41,7 @@
  * filled with NUL characters. If `i` is less than the current length
  * then memory is released to the system.
  *
- * The resulting buffer is guarranteed to be NUL-terminated, i.e.
+ * The resulting buffer is guaranteed to be NUL-terminated, i.e.
  * `!b[appendz(b).i]` will be the case even if both params are 0.
  *
  * @return `i` or -1 if `ENOMEM`

libc/stdio/fgets.c

@@ -27,7 +27,7 @@
  *
  * @param s is output buffer
  * @param size is capacity of s
- * @param f is non-null file oject stream pointer
+ * @param f is non-null file object stream pointer
  * @return s on success, NULL on error, or NULL if EOF happens when
  *     zero characters have been read
  * @see fgets_unlocked()

libc/stdio/fgets_unlocked.c

@@ -28,7 +28,7 @@
  *
  * @param s is output buffer
  * @param size is capacity of s
- * @param f is non-null file oject stream pointer
+ * @param f is non-null file object stream pointer
  * @return s on success, NULL on error, or NULL if EOF happens when
  *     zero characters have been read
  */