fixed config.xml loc for sed command

Dockerfile

@@ -18,7 +18,7 @@ RUN yum update -y
 # Install Folding@home
 RUN rpm -i https://fah.stanford.edu/file-releases/public/release/fahclient/centos-5.3-64bit/v7.3/fahclient-7.3.6-1.x86_64.rpm
 ADD config.xml /etc/fahclient/
-RUN sed -i -e "s/{{USERNAME}}/$USERNAME/;s/{{TEAM}}/$TEAM/;s/{{POWER}}/$POWER/" config.xml
+RUN sed -i -e "s/{{USERNAME}}/$USERNAME/;s/{{TEAM}}/$TEAM/;s/{{POWER}}/$POWER/" /etc/fahclient/config.xml
 RUN chown fahclient:root /etc/fahclient/config.xml
 
 CMD /etc/init.d/FAHClient start && tail -F /var/lib/fahclient/log.txt