Add reference from CachingFileSystem to MMapCache (#933)

Author: djhoese

fsspec/implementations/cached.py

@@ -25,8 +25,8 @@ class CachingFileSystem(AbstractFileSystem):
     access after the initial download. The files are stored in a given
     directory with random hashes for the filenames. If no directory is given,
     a temporary one is used, which should be cleaned up by the OS after the
-    process ends. The files themselves as sparse (as implemented in
-    MMapCache), so only the data which is accessed takes up space.
+    process ends. The files themselves are sparse (as implemented in
+    :class:`~fsspec.core.MMapCache`), so only the data which is accessed takes up space.
 
     Restrictions: