fix(website): fix outdated code in contributor docs

Author: strager

website/public/contribute/create-diagnostic/index.ejs.html

@@ -519,7 +519,7 @@ <h2>3. Detection and reporting of the diagnostic</h2>
 
         <figure>
           <pre><code class="cxx">Source_Code_Span op_span = /* usually this->peek().span */;
-this->diag_reporter_->report(Diag_Comparison_With_Empty_String{
+this->diags_.add(Diag_Comparison_With_Empty_String{
     .comparison_operator = op_span,
 });</code></pre>
         </figure>