fix tutorials latex and links (#261)

tutorials/tutorial3/tutorial.ipynb

@@ -309,7 +309,7 @@
     "\n",
     "A valid option is to impose the initial condition as hard constraint as well. Specifically, our solution is written as:\n",
     "\n",
-    "$$ u_{\\rm{pinn}} = xy(1-x)(1-y)\\cdot NN(x, y, t)\\cdot t + \\cos(\\sqrt{2}\\pi t)sin(\\pi x)\\sin(\\pi y), $$\n",
+    "$$ u_{\\rm{pinn}} = xy(1-x)(1-y)\\cdot NN(x, y, t)\\cdot t + \\cos(\\sqrt{2}\\pi t)\\sin(\\pi x)\\sin(\\pi y), $$\n",
     "\n",
     "Let us build the network first"
    ]