Reference to INSTALL.txt in system.module hook_requirements() should be consistent with installer

During the install phase (install.core.inc), help references to INSTALL.txt use a link – <a href="@install_txt">INSTALL.txt</a>– while the status report in system.install does not. This patch makes system.install consistent with the installer. It's helpful from a usability standpoint since INSTALL.txt doesn't exist in the root directory, and the link takes you directly to the file.