Docs: use a simple clone in the New Beat guide (#6786)

The previous version used `go get` as a trick to clone the repository
in the correct path. However:

* the extra magic can be confusing
* the `go get` results in an error (no buildable Go files). The docs
  were telling the reader to ignore the error, but it seems the wording
  of the error is different now.

I think it's better to use a simple `git clone`, which is what we
use also in the main Contributing guide.

docs/devguide/newbeat.asciidoc

@@ -41,17 +41,16 @@ For general information about contributing to Beats, see <<beats-contributing>>.
 
 After you have https://golang.org/doc/install[installed Go] and set up the
 https://golang.org/doc/code.html#GOPATH[GOPATH] environment variable to point to
-your preferred workspace location, a simple way of getting the source code for
-Beats, including libbeat and the Beat generator, is to do:
+your preferred workspace location, clone the Beats repository in the correct location
+under `GOPATH`:
 
 [source,shell]
 ----------------------------------------------------------------------
-go get github.com/elastic/beats
+mkdir -p ${GOPATH}/src/github.com/elastic
+git clone https://github.com/elastic/beats ${GOPATH}/src/elastic/beats
 ----------------------------------------------------------------------
 
-When you run the command, all source files are downloaded to the
-`$GOPATH/src/github.com/elastic/beats` path. You can ignore the "no buildable Go source files" message because
-you will build the source later. By default `go get`  fetches the master branch. To build your beat
+To build your beat
 on a specific version of libbeat, check out the specific branch ({doc-branch} in the example below):
 
 ["source","sh",subs="attributes"]