Update pip install instructions with quotes for better compatibility.

From issue #852, this is to ensure it works in the upcoming version
of macOS Catalina, and other operating systems/shells.

docs/index.rst

@@ -31,7 +31,7 @@ Clone the repository::
 
 Make a virtualenv, then install the requirements::
 
- pip install -e .[dev]
+ pip install -e ".[dev]"
 
 Start Mu::
 

docs/setup.rst

@@ -15,14 +15,14 @@ Windows, OSX, Linux
 Create a working development environment by installing all the dependencies
 into your virtualenv with::
 
- pip install -e .[dev]
+ pip install -e ".[dev]"
 
 .. note::
 
  The Mu package distribution, as specified in ``setup.py``, declares
  both runtime and extra dependencies.
 
- The above mentioned ``pip install -e .[dev]`` installs all runtime
+ The above mentioned ``pip install -e ".[dev]"`` installs all runtime
  dependencies and most development ones: it should serve nearly everyone.
 
  For the sake of completeness, however, here are a few additional details.
@@ -107,7 +107,7 @@ working development environment:
  Raspberry Pi with::
 
  (mu-venv) $ cd ~/mu-source
- (mu-venv) $ pip install -e .[dev]
+ (mu-venv) $ pip install -e ".[dev]"
 
 7. Run mu::