Add prefix for all commands in documentation (#2877)

* Add prefix for commands in docs

* Add prefix for commands in example's README

docs/tutorial/tests.rst

@@ -28,7 +28,7 @@ Install them both:
 
 .. code-block:: none
 
-    pip install pytest coverage
+    $ pip install pytest coverage
 
 .. _pytest: https://pytest.readthedocs.io/
 .. _coverage: https://coverage.readthedocs.io/
@@ -510,7 +510,7 @@ the test functions you've written.
 
 .. code-block:: none
 
-    pytest
+    $ pytest
 
     ========================= test session starts ==========================
     platform linux -- Python 3.6.4, pytest-3.5.0, py-1.5.3, pluggy-0.6.0
@@ -532,13 +532,13 @@ to run pytest instead of running it directly.
 
 .. code-block:: none
 
-    coverage run -m pytest
+    $ coverage run -m pytest
 
 You can either view a simple coverage report in the terminal:
 
 .. code-block:: none
 
-    coverage report
+    $ coverage report
 
     Name                 Stmts   Miss Branch BrPart  Cover
     ------------------------------------------------------
@@ -553,7 +553,7 @@ An HTML report allows you to see which lines were covered in each file:
 
 .. code-block:: none
 
-    coverage html
+    $ coverage html
 
 This generates files in the ``htmlcov`` directory. Open
 ``htmlcov/index.html`` in your browser to see the report.