base: Add MmapFile as a synonym for MappedFile

I was looking for 'MmapFile' earlier and couldn't find it.  After
being pointed to it, we thought that adding this synonym would help
others trying to find this file.

Test: n/a
Change-Id: I7fbfd8e090f21183a5de3603a76f884562b99c4a
diff --git a/base/include/android-base/mapped_file.h b/base/include/android-base/mapped_file.h
index b719646..2ab49ab 100644
--- a/base/include/android-base/mapped_file.h
+++ b/base/include/android-base/mapped_file.h
@@ -36,7 +36,7 @@
 namespace base {
 
 /**
- * A region of a file mapped into memory.
+ * A region of a file mapped into memory, also known as MmapFile.
  */
 class MappedFile {
  public: