Fix some markdown warnings seen in the Travis CI checks

CppCoreGuidelines.md

@@ -19409,6 +19409,7 @@ A relatively informal definition of terms used in the guidelines
 This is our to-do list.
 Eventually, the entries will become rules or parts of rules.
 Alternatively, we will decide that no change is needed and delete the entry.
+
 * No long-distance friendship
 * Should physical design (what's in a file) and large-scale design (libraries, groups of libraries) be addressed?
 * Namespaces