Fix creating certificate authority doc (apache#3001)

*Motivation*

wget a github link is downloading an html page, not the actual content.
if we want to fetch the actual content, we should use the githubusercontent link.

*Changes*

Change the github link to githubusercontent link.

site2/docs/security-tls-transport.md

@@ -43,7 +43,7 @@ Create a directory for your CA, and place [this openssl configuration file](http
 ```bash
 $ mkdir my-ca
 $ cd my-ca
-$ wget https://github.com/apache/pulsar/tree/master/site2/website/static/examples/openssl.cnf
+$ wget https://raw.githubusercontent.com/apache/pulsar/master/site2/website/static/examples/openssl.cnf
 $ export CA_HOME=$(pwd)
 ```
 

site2/website/versioned_docs/version-2.1.0-incubating/security-tls-transport.md

@@ -44,7 +44,7 @@ Create a directory for your CA, and place [this openssl configuration file](http
 ```bash
 $ mkdir my-ca
 $ cd my-ca
-$ wget https://github.com/apache/incubator-pulsar/tree/master/site2/website/static/examples/openssl.cnf
+$ wget https://raw.githubusercontent.com/apache/pulsar/master/site2/website/static/examples/openssl.cnf
 $ export CA_HOME=$(pwd)
 ```
 

site2/website/versioned_docs/version-2.2.0/security-tls-transport.md

@@ -44,7 +44,7 @@ Create a directory for your CA, and place [this openssl configuration file](http
 ```bash
 $ mkdir my-ca
 $ cd my-ca
-$ wget https://github.com/apache/pulsar/tree/master/site2/website/static/examples/openssl.cnf
+$ wget https://raw.githubusercontent.com/apache/pulsar/master/site2/website/static/examples/openssl.cnf
 $ export CA_HOME=$(pwd)
 ```