Update client version from 2.2 to 2.3 (#5568)

As the latest version of 2.3, the version information needs to be
updated as well.

doc/building.rst

@@ -11,7 +11,7 @@ desktop client.
 .. note:: Build instructions are subject to change as development proceeds.
   Please check the version for which you want to build.
 
-These instructions are updated to work with version 2.2 of the ownCloud Client.
+These instructions are updated to work with version |version| of the ownCloud Client.
 
 Getting Source Code
 -------------------

doc/introduction.rst

@@ -19,7 +19,7 @@ recommended to keep your client updated.
 Improvements and New Features
 -----------------------------
 
-The 2.2 release of the ownCloud desktop sync client has many new features and 
+The |version| release of the ownCloud desktop sync client has many new features and 
 improvements. (See the `complete changelog 
 <https://owncloud.org/changelog/desktop/>`_.)