Do not overwrite earlier command in example

tips/tips.tex

@@ -383,7 +383,7 @@
 
   \begin{lstlisting}[language=none, caption=Setting the global ignore file]
     $ echo "*.out" > ~/.gitignore_global
-    $ echo "testing.txt" > ~/.gitignore_global
+    $ echo "testing.txt" >> ~/.gitignore_global
     $ git config --global core.excludesfile \
       "~/.gitignore_global"
   \end{lstlisting}