remove unnecessary echo from yum docs

Signed-off-by: Patrick Hemmer <patrick.hemmer@gmail.com>

docs/installation/centos.md

@@ -53,7 +53,7 @@ package manager.
 
 3. Add the yum repo.
 
-        $ echo | sudo tee /etc/yum.repos.d/docker.repo <<-EOF
+        $ sudo tee /etc/yum.repos.d/docker.repo <<-EOF
         [dockerrepo]
         name=Docker Repository
         baseurl=https://yum.dockerproject.org/repo/main/centos/7

docs/installation/fedora.md

@@ -48,7 +48,7 @@ There are two ways to install Docker Engine.  You can install with the `yum` pac
 
     For Fedora 21 run:
 
-        $ echo | sudo tee /etc/yum.repos.d/docker.repo <<-EOF
+        $ sudo tee /etc/yum.repos.d/docker.repo <<-EOF
         [dockerrepo]
         name=Docker Repository
         baseurl=https://yum.dockerproject.org/repo/main/fedora/21

docs/installation/oracle.md

@@ -40,7 +40,7 @@ btrfs storage engine on both Oracle Linux 6 and 7.
 
     For version 6:
 
-        $ echo | sudo tee /etc/yum.repos.d/docker.repo <<-EOF
+        $ sudo tee /etc/yum.repos.d/docker.repo <<-EOF
         [dockerrepo]
         name=Docker Repository
         baseurl=https://yum.dockerproject.org/repo/main/oraclelinux/6

docs/installation/rhel.md

@@ -47,7 +47,7 @@ There are two ways to install Docker Engine.  You can install with the `yum` pac
 
 3. Add the yum repo yourself.
 
-  		$ echo | sudo tee /etc/yum.repos.d/docker.repo <<-EOF
+        $ sudo tee /etc/yum.repos.d/docker.repo <<-EOF
         [dockerrepo]
         name=Docker Repository
         baseurl=https://yum.dockerproject.org/repo/main/centos/7