Trivial typo fix (#56)

Author: ujihisa

Diff for: index.html

@@ -217,7 +217,7 @@ <h3 id="test-unit-notify-document">test-unit-notify: Document</h3>
 
         <h2 id="test-unit-rr">test-unit-rr</h2>
         <p>
-          This package provides a <a href="https://github.com/rr/rr">RR</a>, a test dobule library, adapter for test-unit.
+          This package provides a <a href="https://github.com/rr/rr">RR</a>, a test double library, adapter for test-unit.
         </p>
         <h3 id="test-unit-rr-latest">test-unit-rr: The latest release</h3>
         <p>