echo -n to avoid new lines at the end

`echo blah | base64` will actually result in the base64 of `blah\n`, which will usually not be intended by the user.

docs/user-guide/kubectl-cheatsheet.md

@@ -50,8 +50,8 @@ metadata:
   name: mysecret
 type: Opaque
 data:
-  password: $(echo "s33msi4" | base64)
+  password: $(echo -n "s33msi4" | base64)
-  username: $(echo "jane" | base64)
+  username: $(echo -n "jane" | base64)
 EOF
 
 # TODO: kubectl-explain example