Fix typo "phtreads" to "pthreads"

Closes issue #5349

Signed-off-by: Artur Allmann <Artur.Allmann@tptlive.ee>
diff --git a/library/threading.c b/library/threading.c
index 2de117f..5e0aaa4 100644
--- a/library/threading.c
+++ b/library/threading.c
@@ -113,7 +113,7 @@
 int (*mbedtls_mutex_unlock)( mbedtls_threading_mutex_t * ) = threading_mutex_unlock_pthread;
 
 /*
- * With phtreads we can statically initialize mutexes
+ * With pthreads we can statically initialize mutexes
  */
 #define MUTEX_INIT  = { PTHREAD_MUTEX_INITIALIZER, 1 }