Misspell correction (#826)

Did a `misspell . | grep -v ^vendor` and fixed
several typos.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

middleware/cache/freq/freq.go

@@ -7,7 +7,7 @@ import (
 	"time"
 )
 
-// Freq tracks the frequence of things.
+// Freq tracks the frequencies of things.
 type Freq struct {
 	// Last time we saw a query for this element.
 	last time.Time

middleware/cache/handler.go

@@ -43,7 +43,7 @@ func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 
 		if c.prefetch > 0 && i.Freq.Hits() > c.prefetch && pct < c.percentage {
 			// When prefetching we loose the item i, and with it the frequency
-			// that we've gathered sofar. See we copy the frequence info back
+			// that we've gathered sofar. See we copy the frequencies info back
 			// into the new item that was stored in the cache.
 			prr := &ResponseWriter{ResponseWriter: w, Cache: c, prefetch: true}
 			middleware.NextOrFailure(c.Name(), c.Next, ctx, prr, r)