Fix instructions for checking out ohos repositories

repo defaults to checking out the "master" branch, but manifest.git doesn't have one - it uses a "develop" branch instead.

Signed-off-by: Bernhard Rosenkränzer <bernhard.rosenkraenzer.ext@huawei.com>

documentation/source/quick-start/build-open-harmony.rst

@@ -33,7 +33,7 @@ Once git repo has been installed as mentioned in prerequisite, clone the necessa
  .. code-block:: console
  
       $ mkdir ohos; cd ohos
-      $ repo init -u https://git.ostc-eu.org/OSTC/manifest.git
+      $ repo init -u https://git.ostc-eu.org/OSTC/manifest.git -b develop
       $ repo sync
 
  Above commands creates the following directory structure: