Fix links to Pyomo docs

Author: leonlan

notebooks/01/02-production-planning-basic.ipynb

@@ -25,11 +25,11 @@
     "\n",
     "This notebook introduces basic elements of Pyomo common to most applications for the small production planning problem introduced earlier in [this notebook](01-production-planning.ipynb) introduced in a companion notebook:\n",
     "\n",
-    "* [Variables](https://pyomo.readthedocs.io/en/latest/pyomo_modeling_components/Variables.html)\n",
-    "* [Expressions](https://pyomo.readthedocs.io/en/latest/pyomo_modeling_components/Expressions.html)\n",
-    "* [Objectives](https://pyomo.readthedocs.io/en/latest/pyomo_modeling_components/Objectives.html)\n",
-    "* [Constraints](https://pyomo.readthedocs.io/en/latest/pyomo_modeling_components/Constraints.html)\n",
-    "* [SolverFactory](https://pyomo.readthedocs.io/en/stable/solving_pyomo_models.html)\n",
+    "* [Variables](https://pyomo.readthedocs.io/en/6.8.0/pyomo_modeling_components/Variables.html)\n",
+    "* [Expressions](https://pyomo.readthedocs.io/en/6.8.0/pyomo_modeling_components/Expressions.html)\n",
+    "* [Objectives](https://pyomo.readthedocs.io/en/6.8.0/pyomo_modeling_components/Objectives.html)\n",
+    "* [Constraints](https://pyomo.readthedocs.io/en/6.8.0/pyomo_modeling_components/Constraints.html)\n",
+    "* [SolverFactory](https://pyomo.readthedocs.io/en/6.8.0/solving_pyomo_models.html)\n",
     "\n",
     "The Pyomo model shown below is a direct translation of the mathematical model into basic Pyomo components. In this approach, parameter values from the mathematical model are included directly in the Pyomo model for simplicity. This method works well for problems with a few decision variables and constraints, but it limits the reuse of the model. Another notebook will demonstrate Pyomo features for writing models for more generic, \"data-driven\" applications.\n",
     "\n",

notebooks/01/03-production-planning-advanced.ipynb

@@ -26,8 +26,8 @@
     "\n",
     "The additional Pyomo components used in this notebook are:\n",
     "\n",
-    "* [Sets](https://pyomo.readthedocs.io/en/latest/pyomo_modeling_components/Sets.html)\n",
-    "* [Parameters](https://pyomo.readthedocs.io/en/latest/pyomo_modeling_components/Parameters.html)\n",
+    "* [Sets](https://pyomo.readthedocs.io/en/6.8.0/pyomo_modeling_components/Sets.html)\n",
+    "* [Parameters](https://pyomo.readthedocs.io/en/6.8.0/pyomo_modeling_components/Parameters.html)\n",
     "\n",
     "These components enable the use indexed variables and constraints. The combination of sets and indices are essential to building scalable and maintainable models for more complex applications. \n",
     "\n",