fix some 404 links to doc site

Author: SethTisue

_includes/downloads-scala2.html

@@ -9,7 +9,7 @@ <h2>Ways to Install This Release</h2>
     <ul>
       <li>
         Using <a
-          href="https://docs.scala-lang.org/getting-started/index.html#using-the-scala-installer-recommended-way">Coursier
+          href="https://docs.scala-lang.org/getting-started/install-scala.html#install-scala-on-your-computer">Coursier
           CLI</a>, run:<br>
         <code>cs install scala:{{ page.release_version }} && cs install scalac:{{ page.release_version }}</code>
       </li>

_includes/downloads-scala3.html

@@ -12,7 +12,7 @@ <h2>Ways to Install This Release</h2>
     <ul>
       <li>
         Using <a
-          href="https://docs.scala-lang.org/getting-started/index.html#using-the-scala-installer-recommended-way">Coursier
+          href="https://docs.scala-lang.org/getting-started/install-scala.html#install-scala-on-your-computer">Coursier
           CLI</a>, run:<br>
         <code>cs install scala:{{page.release_version}} && cs install scalac:{{page.release_version}}</code>
       </li>