fix typo

Author: RubyFore

_includes/footer.html

@@ -1,7 +1,7 @@
     <!-- Footer -->
     <footer>
         <div class="container text-center">
-            <p>Based on <a href="https://justinmae.github.io/grayscale-theme/">Grayscale Bootstraph theme</a>, powered by Jekyll</p>
+            <p>Based on <a href="https://justinmae.github.io/grayscale-theme/">Grayscale Bootstrap theme</a>, powered by Jekyll</p>
             <!--- <p> {{ site.footer }} 20{{ site.time | date: '%y' }}</p>--->
         </div>
     </footer>