Fix typo: add missing 'a'

'...to a friend on -a- different operating system...'

Author: jasonflorentino

chapter4_interactive_prompt.html

@@ -186,7 +186,7 @@ <h3>Compiling with Editline</h3>
 
 <h2 id='the_c_preprocessor'>The C Preprocessor</h2> <hr/>
 
-<p>For such a small project it might be okay that we have to program differently depending on what operating system we are using, but if I want to send my source code to a friend on different operating system to give me a hand with the programming, it is going to cause problems. In an ideal world I'd wish for my source code to be able to compile no matter where, or on what computer, it is being compiled. This is a general problem in C, and it is called <em>portability</em>. There is not always an easy or correct solution.</p>
+<p>For such a small project it might be okay that we have to program differently depending on what operating system we are using, but if I want to send my source code to a friend on a different operating system to give me a hand with the programming, it is going to cause problems. In an ideal world I'd wish for my source code to be able to compile no matter where, or on what computer, it is being compiled. This is a general problem in C, and it is called <em>portability</em>. There is not always an easy or correct solution.</p>
 
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   <img src="/static/img/octopus.png" alt="octopus" class="img-responsive" width="266px" height="268px"/>