Update installation.md

Fixed the MySQL documentation link.

doc/install/installation.md

@@ -144,7 +144,7 @@ GitLab Shell is an ssh access and repository management software developed speci
 # 5. Database
-We recommend using a PostgreSQL database. For MySQL check [MySQL setup guide](doc/install/database_mysql.md).
+We recommend using a PostgreSQL database. For MySQL check [MySQL setup guide](database_mysql.md).
     # Install the database packages
     sudo apt-get install -y postgresql-9.1 postgresql-client libpq-dev