Fix some typos in comment (#2520)

Signed-off-by: zhoulin xie <zhoulin.xie@daocloud.io>

core/dnsserver/address.go

@@ -72,7 +72,7 @@ func SplitProtocolHostPort(address string) (protocol string, ip string, port str
 
 type zoneOverlap struct {
 	registeredAddr map[zoneAddr]zoneAddr // each zoneAddr is registered once by its key
-	unboundOverlap map[zoneAddr]zoneAddr // the "no bind" equiv ZoneAdddr is registered by its original key
+	unboundOverlap map[zoneAddr]zoneAddr // the "no bind" equiv ZoneAddr is registered by its original key
 }
 
 func newOverlapZone() *zoneOverlap {

plugin.md

@@ -106,7 +106,7 @@ entire domain and all sub domains.
 In this example the *file* plugin is handling all names below (and including) `example.org`. If
 a query comes in that is not a subdomain (or equal to) `example.org` the next plugin is called.
 
-Now, the world isn't perfect, and there are good reasons to "fallthrough" to the next middlware,
+Now, the world isn't perfect, and there are good reasons to "fallthrough" to the next middleware,
 meaning a plugin is only responsible for a *subset* of names within the zone. The first of these
 to appear was the *reverse* plugin that synthesis PTR and A/AAAA responses (useful with IPv6).
 

plugin/autopath/autopath.go

@@ -20,7 +20,7 @@ through the search path.
 
 It is assume the search path ordering is identical between server and client.
 
-Midldeware implementing autopath, must have a function called `AutoPath` of type
+Middleware implementing autopath, must have a function called `AutoPath` of type
 autopath.Func. Note the searchpath must be ending with the empty string.
 
 I.e: