target GLM_linear >> glm_linear (#6714)

Author: reshamas

docs/source/learn/core_notebooks/GLM_linear.ipynb

@@ -4,7 +4,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "(GLM_linear)=\n",
+    "(glm_linear)=\n",
     "# GLM: Linear regression\n",
     "\n",
     "This tutorial is adapted from a [blog post by Thomas Wiecki called \"The Inference Button: Bayesian GLMs made easy with PyMC\"](http://twiecki.github.io/blog/2013/08/12/bayesian-glms-1/).\n",