Fixes minor typos in comments in pk.h and ctr_drbg.c

Fixes typos in PRs #475 and #437
diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index aefddfa..6962d68 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -67,7 +67,7 @@
 }
 
 /*
- * Non-public function wrapped by ctr_crbg_init(). Necessary to allow NIST
+ * Non-public function wrapped by mbedtls_ctr_drbg_init(). Necessary to allow NIST
  * tests to succeed (which require known length fixed entropy)
  */
 int mbedtls_ctr_drbg_seed_entropy_len(