Fix incorrect CLI option name

Commit 56d8957 broke the docs by changing the wrong place.

index.html

@@ -36,7 +36,7 @@ <h2 id="project_tagline">A simple tool for visually comparing two PDF files</h2>
 
 <p>It takes two PDF files as arguments. By default, its only output is its return
 code, which is 0 if there are no differences and 1 if the two PDFs differ. If
-given the <code>--pdf</code> option, <em>output-diff</em> produces a PDF file with
+given the <code>--output-diff</code> option, <em>diff-pdf</em> produces a PDF file with
 visually highlighted differences:</p>
 
 <pre><code>$ diff-pdf --output-diff=diff.pdf a.pdf b.pdf