Fix minor typo in Making a Custom Statespace Model.ipynb. (pymc-devs#417)

Change x_{t+1} to x_t on the left-hand side and x_t to x_{t-1} on the
right-hand side of eq (1).

Author: niskrev

notebooks/Making a Custom Statespace Model.ipynb

@@ -73,7 +73,7 @@
     "\n",
     "The statespace module is designed to make it easy for users to create their own statespace models. At its core, a statspace model is just a system of two linear equatons:\n",
     "\n",
-    "$$\\begin{align} x_{t+1} &= A_t x_t + c_t + R_t \\varepsilon_t, & \\varepsilon_t &\\sim N(0, Q_t) \\\\\n",
+    "$$\\begin{align} x_{t} &= A_t x_{t-1} + c_t + R_t \\varepsilon_t, & \\varepsilon_t &\\sim N(0, Q_t) \\\\\n",
     "y_t &= Z_t x_t + d_t + \\eta_t, & \\eta_t &\\sim N(0, H_t) \\\\\n",
     "x_0 &\\sim N(\\bar x, P)\\end{align}$$\n",
     "\n",