respect proper numpydoc section name

Because of the incorrect name, the example in the docstring of lamdify
was not even visible in the documentation.

kwant/continuum/_common.py

@@ -63,7 +63,7 @@ def lambdify(expr, locals=None):
     locals : dict or ``None`` (default)
         Additional definitions for `~kwant.continuum.sympify`.
 
-    Example:
+    Examples
     --------
         >>> f = lambdify('a + b', locals={'b': 'b + c'})
         >>> f(1, 3, 5)
@@ -226,8 +226,8 @@ def monomials(expr, gens=None):
     -------
     dictionary (generator: monomial)
 
-    Example
-    -------
+    Examples
+    --------
         >>> expr = kwant.continuum.sympify("A * (x**2 + y) + B * x + C")
         >>> monomials(expr, gens=('x', 'y'))
         {1: C, x: B, x**2: A, y: A}

kwant/solvers/sparse.py

@@ -32,11 +32,6 @@ if uses_umfpack:
         Return a fuction for solving a sparse linear system, with A
         pre-factorized.
 
-        Example:
-        solve = factorized(A) # Makes LU decomposition.
-        x1 = solve(rhs1) # Uses the LU factors.
-        x2 = solve(rhs2) # Uses again the LU factors.
-
         Parameters
         ----------
         A : csc_matrix
@@ -49,6 +44,12 @@ if uses_umfpack:
             0.001 respectively. Whether piv_tol or sym_piv_tol are used is
             decided internally by UMFPACK, depending on whether the matrix is
             "symmetric" enough.
+
+        Examples
+        --------
+        solve = factorized(A) # Makes LU decomposition.
+        x1 = solve(rhs1) # Uses the LU factors.
+        x2 = solve(rhs2) # Uses again the LU factors.
         """
 
         if not sp.isspmatrix_csc(A):