update workflow image (#701)

Author: GiovanniCanali

tutorials/static/pina_wokflow.png

@@ -40,7 +40,7 @@
     "## The PINA Workflow \n",
     "\n",
     "<p align=\"center\">\n",
-    "    <img src=\"http://raw.githubusercontent.com/mathLab/PINA/master/tutorials/static/pina_wokflow.png\" alt=\"PINA Workflow\" width=\"1000\"/>\n",
+    "    <img src=\"http://raw.githubusercontent.com/mathLab/PINA/master/tutorials/static/pina_workflow.png\" alt=\"PINA Workflow\" width=\"1000\"/>\n",
     "</p>\n",
     "\n",
     "Solving a differential problem in **PINA** involves four main steps:\n",