composer.xml Fix the bad link (remove the extra slash) (#4159)

Author: mmalferov

install/composer.xml

@@ -27,7 +27,7 @@
    Briefly, this is done by creating a <literal>composer.json</literal>
    in the project, using Composer to install the latest version of the
    package, and including Composer's autoload script to make it available
-   to the code. The <link xlink:href="&url.composer;/doc/01-basic-usage.md">Composer
+   to the code. The <link xlink:href="&url.composer;doc/01-basic-usage.md">Composer
    "Basic Usage" documentation</link> goes into this in more depth.
   </simpara>
   <example>