DOC: Align the code blocks in CONTRIBUTING.md with tab

This removes the spaces which were appearing at the start of the
code on each line in CONTRIBUTING.md by aligning the code text
with the relevant tab (4 space) indentation level.

Author: scottclowe

CONTRIBUTING.md

@@ -21,24 +21,24 @@ GitHub:
 
 2. Clone this copy to your local disk:
 
-          $ git clone [email protected]:YourLogin/scikit-learn.git
-          $ cd scikit-learn
+        $ git clone [email protected]:YourLogin/scikit-learn.git
+        $ cd scikit-learn
 
 3. Create a branch to hold your changes:
 
-          $ git checkout -b my-feature
+        $ git checkout -b my-feature
 
    and start making changes. Never work in the ``master`` branch!
 
 4. Work on this copy on your computer using Git to do the version
    control. When you're done editing, do:
 
-          $ git add modified_files
-          $ git commit
+        $ git add modified_files
+        $ git commit
 
    to record your changes in Git, then push them to GitHub with:
 
-          $ git push -u origin my-feature
+        $ git push -u origin my-feature
 
 Finally, go to the web page of your fork of the scikit-learn repo,
 and click 'Pull request' to send your changes to the maintainers for
@@ -56,7 +56,7 @@ following rules before submitting a pull request:
 -  All other tests pass when everything is rebuilt from scratch. On
    Unix-like systems, check with (from the toplevel source folder):
 
-          $ make
+        $ make
 
 -  When adding additional functionality, provide at least one
    example script in the ``examples/`` folder. Have a look at other
@@ -79,23 +79,23 @@ tools:
 
 -  Code with good unittest coverage (at least 80%), check with:
 
-          $ pip install nose coverage
-          $ nosetests --with-coverage path/to/tests_for_package
+        $ pip install nose coverage
+        $ nosetests --with-coverage path/to/tests_for_package
 
 -  No pyflakes warnings, check with:
 
-           $ pip install pyflakes
-           $ pyflakes path/to/module.py
+        $ pip install pyflakes
+        $ pyflakes path/to/module.py
 
 -  No PEP8 warnings, check with:
 
-           $ pip install pep8
-           $ pep8 path/to/module.py
+        $ pip install pep8
+        $ pep8 path/to/module.py
 
 -  AutoPEP8 can help you fix some of the easy redundant errors:
 
-           $ pip install autopep8
-           $ autopep8 path/to/pep8.py
+        $ pip install autopep8
+        $ autopep8 path/to/pep8.py
 
 Bonus points for contributions that include a performance analysis with
 a benchmark script and profiling output (please report on the mailing