Fix typo in image alt text.

08-Placing-Barriers-in-the-Simulation-Box/01-Barriers.ipynb

@@ -54,7 +54,7 @@
    "source": [
     "Similarly, HOOMD-blue _always_ applies pairwise interactions accounting for the periodic boundary conditions. When desired, set the box length and width of the disallowed region accordingly to prevent such interactions:\n",
     "\n",
-    "![Example of an invalid barrier](valid-barrier.svg)\n",
+    "![Example of a valid barrier](valid-barrier.svg)\n",
     "\n",
     "In the above example, the barriers are half the interaction distance away from the edge of the primary box image. Therefore, a particle who's center is exactly on the edge of the allowed region can never interact with a particle in the allowed region on the opposite side of the box."
    ]