DOC: make conda cmd in doc exact cmd needed for full install

I think people might miss the warning text and just copy the listed conda cmd,
so having this cmd just work could avoid some issues.

docs/source/installation.rst

@@ -17,14 +17,14 @@ Installing PyDM and Prerequisites with Anaconda
 After installing Anaconda (see https://www.anaconda.com/download/), create a new
 environment for PyDM::
   
-  $ conda create -n pydm-environment python=3.8 pyqt=5 pip numpy scipy six psutil pyqtgraph pydm -c conda-forge
+  $ conda create -n pydm-environment python=3.8 pyqt=5.12.3 pip numpy scipy six psutil pyqtgraph pydm -c conda-forge
   $ source activate pydm-environment
 
 .. warning::
     There is currently no PyQt 5.15+ build available on conda or PyPI that has
     designer support for python plugins.
 
-    In order to use PyDM widgets in designer, please make sure to pin the PyQt version to 5.12.3 or lower
+    In order to use PyDM widgets in designer, we make sure to pin the PyQt version to 5.12.3 or lower
     until this is resolved.
 
 Once you've installed and activated the environment, you should be able to run 'pydm' to launch PyDM, or run 'designer' to launch Qt Designer.  If you are on Windows, run these commands from the Anaconda Prompt.