Update build.md with correct description

Author: rayou

docs/build.md

@@ -37,7 +37,7 @@ Visit the page for the pull request and look at the end of the url for the PR nu
 https://github.com/BoostIO/Boostnote/pull/2794
 </pre>
 In the following, replace \<PR> with that number (no brackets).
-For the above url, you would replace \<PR> with 2794
+For URLs below, you would replace \<PR> with 2794
 
 _If you do not have a local copy of the master branch yet_
 ```