Fix on-place -> in-place

Closes #1373

src/sds.c

@@ -109,7 +109,7 @@ void sdsupdatelen(sds s) {
     sh->len = reallen;
 }
 
-/* Modify an sds string on-place to make it empty (zero length).
+/* Modify an sds string in-place to make it empty (zero length).
  * However all the existing buffer is not discarded but set as free space
  * so that next append operations will not require allocations up to the
  * number of bytes previously available. */