Fix typo on kubectl download for Mac

Closes: #171, thanks @brp-magnusd

Author: alexellis

lab1b.md

@@ -19,7 +19,7 @@ mv kubectl /usr/local/bin/
 
 ```sh
 export VER=$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt)
-curl -LO https://storage.googleapis.com/kubernetes-release/release/$VER/bin/darwin/amd64/kubectl.exe
+curl -LO https://storage.googleapis.com/kubernetes-release/release/$VER/bin/darwin/amd64/kubectl
 chmod +x kubectl
 mv kubectl /usr/local/bin/
 ```