CLEANUP: Fix typos in the cache subsystem

Fix common misspells in the code comments of the cache subsystem.
diff --git a/src/cache.c b/src/cache.c
index 9b77121..2157433 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -218,7 +218,7 @@
 		return len;
 
 	if (!len) {
-		/* Nothing to foward */
+		/* Nothing to forward */
 		ret = len;
 	}
 	else if (st->hdrs_len >= len) {
@@ -565,7 +565,7 @@
 
 /*
  * Append an "Age" header into <chn> channel for this <ce> cache entry.
- * This is the responsability of the caller to insure there is enough
+ * This is the responsibility of the caller to insure there is enough
  * data in the channel.
  *
  * Returns the number of bytes inserted if succeeded, 0 if failed.
@@ -642,7 +642,7 @@
 	if (unlikely(si->state == SI_ST_DIS || si->state == SI_ST_CLO))
 		goto out;
 
-	/* Check if the input buffer is avalaible. */
+	/* Check if the input buffer is available. */
 	if (res->buf.size == 0) {
 		/* buf.size==0 means we failed to get a buffer and were
 		 * already subscribed to a wait list to get a buffer.