Comment fix.

src/patricia.c

@@ -47,7 +47,7 @@
  * PT_* functions.
  *
  * idx is an index to the strings array, and the region of the string
- * represented at a node is strings[idx][off] to strings[idx][len],
+ * represented at a node is strings[idx][off] to strings[idx][off+len],
  * inclusive.
  *
  * Terminating nulls form a part of the target string. So "foo\0" and