fixed typo activity content (#258)

Problem
=======

1. `Update` announcement type should be included in `Note` Activity
content table
2. in `Note` activity content example there was a `summary` field which
is not part of acceptable fields

.gitignore

@@ -2,3 +2,4 @@
 .vscode
 node_modules/
 book
+.DS_Store

pages/ActivityContent/Overview.md

@@ -16,7 +16,7 @@ DSNP uses only the following content types at the root level:
 
 | Name | Description | DSNP Announcements |
 | ---- | ----------- | -------------------- |
-[Note](Types/Note.md) | standard user content | [Broadcast](../DSNP/Types/Broadcast.md), [Reply](../DSNP/Types/Reply.md) |
+[Note](Types/Note.md) | standard user content | [Broadcast](../DSNP/Types/Broadcast.md), [Reply](../DSNP/Types/Reply.md), [Update](../DSNP/Types/Update.md) |
 [Profile](Types/Profile.md) | user profile content | [Profile](../DSNP/Types/Profile.md) |
 
 ## Associated Types

pages/ActivityContent/Types/Note.md

@@ -38,7 +38,6 @@
   "type": "Note",
   "content": "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.",
   "mediaType": "text/plain",
-  "summary": "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
   "attachment": [
     {
       "type": "Link",