fix internal links in markdown doc files

doc/mef_library_differences.md

@@ -38,4 +38,4 @@ NuGet MEF attributes as only they can describe sharing boundaries.
   VS MEF offers no such facility. The catalog must be complete before the graph is created.
   VS-MEF has [compatible equivalents for two common uses of `CompositionContainer.Compose`][Compose].
 
-[Compose]: doc/dynamic_recomposition.md
+[Compose]: dynamic_recomposition.md

doc/why.md

@@ -38,4 +38,4 @@ applications already.
 Learn more about [the differences between .NET MEF, NuGet MEF, and this library][Differences].
 
 [MEFv2Pkg]: https://www.nuget.org/packages/system.composition
-[Differences]: doc/mef_library_differences.md
+[Differences]: mef_library_differences.md