title: Replace only the first instance of a substring in an NSString
body: <p>So if you have an NSString that goes:</p>
  
  <pre><code>@"My blue car is bigger than my blue shoes or my blue bicycle";
  </code></pre>
  
  <p>I would like a method that replaces only the first instance of blue with green, to produce:</p>
  
  <pre><code>@"My green car is bigger than my blue shoes or my blue bicycle";
  </code></pre>
  
  <p>How does one do this?</p>