sympy/simplify: Reindent docstring of sqrtdenest()

The References section of the sqrtdenest() docstring wasn't rendering
correctly in Sphinx documentation; fix this by indenting it correctly.
This also fixes a warning thrown by Sphinx.

sympy/simplify/sqrtdenest.py

@@ -120,8 +120,9 @@ def sqrtdenest(expr, max_iter=3):
     References
     ==========
     [1] http://www.almaden.ibm.com/cs/people/fagin/symb85.pdf
+
     [2] D. J. Jeffrey and A. D. Rich, 'Symplifying Square Roots of Square Roots
-        by Denesting' (available at http://www.cybertester.com/data/denest.pdf)
+    by Denesting' (available at http://www.cybertester.com/data/denest.pdf)
 
     """
     expr = expand_mul(sympify(expr))