Fix several typos in docs. (#426)

Fixes several typos in docs.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

middleware/cache/README.md

@@ -26,7 +26,7 @@ cache [TTL] [ZONES...] {
 ~~~
 
 * **TTL**  and **ZONES** as above.
-* `success`, override the settings for caching succesful responses, **CAPACITY** indicates the maximum
+* `success`, override the settings for caching successful responses, **CAPACITY** indicates the maximum
   number of packets we cache before we start evicting (LRU). **TTL** overrides the cache maximum TTL.
 * `denial`, override the settings for caching denial of existence responses, **CAPACITY** indicates the maximum
   number of packets we cache before we start evicting (LRU). **TTL** overrides the cache maximum TTL.

middleware/kubernetes/README.md

@@ -16,7 +16,7 @@ The record name format can be changed by specifying a name template in the Coref
 kubernetes [ZONES...]
 ~~~
 
-* `ZONES` zones kubernetes should be authorative for. Overlapping zones are ignored.
+* `ZONES` zones kubernetes should be authoritative for. Overlapping zones are ignored.
 
 
 Or if you want to specify an endpoint:

middleware/kubernetes/SkyDNS.md

@@ -27,7 +27,7 @@ an entry: `1-2-3-4.default.pod.cluster.local.`
 ####A Records and hostname Based on Pod Annotations - A Beta Feature in Kubernetes v1.2
 Currently when a pod is created, its hostname is the Pod's `metadata.name` value.
 With v1.2, users can specify a Pod annotation, `pod.beta.kubernetes.io/hostname`, to specify what the Pod's hostname should be.
-If the annotation is specified, the annotation value takes precendence over the Pod's name, to be the hostname of the pod.
+If the annotation is specified, the annotation value takes precedence over the Pod's name, to be the hostname of the pod.
 For example, given a Pod with annotation `pod.beta.kubernetes.io/hostname: my-pod-name`, the Pod will have its hostname set to "my-pod-name".
 
 v1.2 introduces a beta feature where the user can specify a Pod annotation, `pod.beta.kubernetes.io/subdomain`, to specify what the Pod's subdomain should be.