"Remove extra :"

The extra colon caused sphinx to expect a literal block, but it was in
fact just a typo

showcase/openfe_showcase.ipynb

@@ -1214,7 +1214,7 @@
     "### Defining the RBFE simulation settings\n",
     "\n",
     "There are various different parameters which can be set to determine\n",
-    "how the RBFE simulation will take place. To allow for maximum user flexibility, these are defined as a series of settings objects which control the following::\n",
+    "how the RBFE simulation will take place. To allow for maximum user flexibility, these are defined as a series of settings objects which control the following:\n",
     "\n",
     "1. `protocol_repeats`: The number of completely independent repeats of the entire sampling process.\n",
     "\n",