Fixed another typo

_includes/sinatra-respond-with.html

@@ -63,7 +63,7 @@ <h1>Sinatra::RespondWith</h1>
 <h2>Security</h2>
 <p>
 Since methods are triggered based on client input, this can lead to
-security issues (but not as severe as those might apear in the first
+security issues (but not as severe as those might appear in the first
 place: keep in mind that only known file extensions are used). You
 therefore should limit the possible formats you serve.
 </p>