Fix example code.

llvm-svn: 40493
diff --git a/llvm/docs/WritingAnLLVMPass.html b/llvm/docs/WritingAnLLVMPass.html
index c967000..58bea1a 100644
--- a/llvm/docs/WritingAnLLVMPass.html
+++ b/llvm/docs/WritingAnLLVMPass.html
@@ -320,6 +320,7 @@
     }
   };
   
+  char Hello::ID = 0;
   RegisterPass&lt;Hello&gt; X("<i>hello</i>", "<i>Hello World Pass</i>");
 }
 </pre></div>