Fix exported SSL_CERT_FILE variable (#28968)

The variable was pointing to a file with a `.cer` extension but the curl command downloads a `.pem` file which makes executions of the bash script fail

docs/docsite/rst/guide_gce.rst

@@ -168,7 +168,7 @@ For the following use case, let's use this small shell script as a wrapper.
     exit 1
   fi
 
-  export SSL_CERT_FILE=$(pwd)/cacert.cer
+  export SSL_CERT_FILE=$(pwd)/cacert.pem
   export ANSIBLE_HOST_KEY_CHECKING=False
 
   if [[ ! -f "$SSL_CERT_FILE" ]]; then