remove references to Rasa X in install

I missed a couple mentions of Rasa X in RasaHQ#4946

Author: TyDunn

docs/user-guide/installation.rst

@@ -1,5 +1,5 @@
-:desc: Manage our open source NLU on premise to allow local intent recognition,
-       entity extraction and customisation of the language models.
+:desc: Install Rasa Open Source on premises to enable local and customizable
+       Natural Lanaguage Understanding and Dialogue Management.
 :meta_image: https://i.imgur.com/nGF1K8f.jpg
 
 .. _installation:
@@ -13,7 +13,7 @@ Installation
 Quick Installation
 ~~~~~~~~~~~~~~~~~~
 
-You can install both Rasa Open Source and Rasa X using pip (requires Python 3.6 or 3.7).
+You can install Rasa Open Source using pip (requires Python 3.6 or 3.7).
 
 .. code-block:: bash
 
@@ -96,8 +96,8 @@ Otherwise, proceed with the instructions below to install them.
             C:\> pip3 install -U pip
 
 .. note::
-   Note that `pip` in this refers to `pip3` as Rasa requires python3. To see which version the `pip` 
-   command on your machine calls use `pip --version`.
+   Note that `pip` in this refers to `pip3` as Rasa Open Source requires python3. To see which version 
+   the `pip` command on your machine calls use `pip --version`.
 
 
 2. Create a virtual environment (strongly recommended)
@@ -149,13 +149,13 @@ Tools like `virtualenv <https://virtualenv.pypa.io/en/latest/>`_ and `virtualenv
 
             $ pip install -U pip
 
-        To install Rasa Open Source without Rasa X:
+        To install Rasa Open Source:
 
         .. code-block:: bash
 
             $ pip install rasa
 
-**Congratulations! You have successfully installed Rasa!**
+**Congratulations! You have successfully installed Rasa Open Source!**
 
 You can now head over to the tutorial.