test/README: Update the CNI plugins instructions for /cni -> /plugins

Catching up with the Dockerfile change from f51b0a1 (Dockerfile: move
to containernetworking/plugins, 2017-05-25, cri-o#536).  The new plugins
commit from f51b0a1 is still the current Dockerfile entry.

This commit also replaces the previous 'go get' call with a git clone
to match the Dockerfile's approach.  I've added an additional 'cd'
call so I don't have to repeat $GOPATH/... more than once, but other
than that, the example matches the current Dockerfile entry.

I've also removed some line-continuation slashes we've been dragging
around since the section landed 07ccda3 (tests: Install CNI
configuration files by default, 2017-04-06, cri-o#434).  I'm guessing they
were a copy/paste bug from the Dockerfile, but this example has new
prompts for each command (so it doesn't need continuation) while the
Dockerfile is using && chaining (so it does).

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

test/README.md

@@ -41,11 +41,12 @@ You will also need to install the [CNI](https://github.com/containernetworking/c
 the the default pod test template runs without host networking:
 
 ```
-$ go get github.com/containernetworking/cni
-$ cd "$GOPATH/src/github.com/containernetworking/cni"
-$ git checkout -q d4bbce1865270cd2d2be558d6a23e63d314fe769
-$ ./build.sh \
-$ mkdir -p /opt/cni/bin \
+$ cd "$GOPATH/src/github.com/containernetworking"
+$ git clone https://github.com/containernetworking/plugins.git
+$ cd plugins
+$ git checkout -q dcf7368eeab15e2affc6256f0bb1e84dd46a34de
+$ ./build.sh
+$ mkdir -p /opt/cni/bin
 $ cp bin/* /opt/cni/bin/
 ```