Fix docs build due to an invalid link (#4389)

build/mkdocs/Dockerfile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM alpine:3.9
+FROM alpine:3.10
 
 RUN apk update && apk add --no-cache \
   bash \

docs/examples/psp/README.md

@@ -15,9 +15,9 @@ kubectl apply -f https://raw.githubusercontent.com/kubernetes/ingress-nginx/mast
 ```
 
 Now that the pod security policy is applied, we can continue as usual by applying the
-[mandatory.yaml](../../../deploy/static/mandatory.yaml)
+[mandatory.yaml](https://raw.githubusercontent.com/kubernetes/ingress-nginx/master/deploy/static/mandatory.yaml)
 according to the [Installation Guide](../../deploy/index.md). 
 
 Note: PSP permissions must be granted before to the creation of the Deployment and the ReplicaSet.
 If the Deployment or ReplicaSet already exist, they will receive the PSP permissions
-only after deleting them and reapplying mandatory.yaml.
+only after deleting them and reapplying mandatory.yaml.

requirements-docs.txt

@@ -1,4 +1,4 @@
-mkdocs-material~=4.0.2
+mkdocs-material~=4.4.0
 mkdocs~=1.0.4
 pymdown-extensions~=6.0
 pygments~=2.3.1