Fix typos in doc and comments

Changes persistant -> persistent. www.dictionary.com does not know
persistant (with an A), but should it be one of those things you can
spell in more than one correct way, let me know.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
diff --git a/lib/textsearch.c b/lib/textsearch.c
index 2cb4a43..98bcadc 100644
--- a/lib/textsearch.c
+++ b/lib/textsearch.c
@@ -40,7 +40,7 @@
  *       configuration according to the specified parameters.
  *   (3) User starts the search(es) by calling _find() or _next() to
  *       fetch subsequent occurrences. A state variable is provided
- *       to the algorihtm to store persistant variables.
+ *       to the algorihtm to store persistent variables.
  *   (4) Core eventually resets the search offset and forwards the find()
  *       request to the algorithm.
  *   (5) Algorithm calls get_next_block() provided by the user continously