Fix get_stress() example and fix pypi readme image

Author: robbievanleeuwen

README_pypi.rst

@@ -1,4 +1,4 @@
-.. image:: logo.png
+.. image:: https://raw.githubusercontent.com/robbievanleeuwen/section-properties/master/logo.png
   :width: 100 %
   :alt: sectionproperties
   :align: left

sectionproperties/analysis/cross_section.py

@@ -2566,8 +2566,8 @@ def get_stress(self):
             stress_post = section.calculate_stress(N=10e3)
 
             stresses = stress_post.get_stress()
-            print('Material {0}.format(stresses[0]['Material']))
-            print('Axial Stresses {0}'.format(stress[0]['sig_zz_n']))
+            print('Material: {0}'.format(stresses[0]['Material']))
+            print('Axial Stresses: {0}'.format(stresses[0]['sig_zz_n']))
 
             $ Material: default
             $ Axial Stresses: [3.64025694 3.64025694 3.64025694 ... 3.64025694 3.64025694 3.64025694]