Fix error in xml manifest doc.

The docs on the annotations say that zero or more may exist as a child
of a project, so that means that a "*" instead of a "?" should be used.

Change-Id: Iff855d003dfb05cd980f285a237332914e1dad70

docs/manifest-format.txt

@@ -47,7 +47,7 @@ following DTD:
     <!ELEMENT manifest-server (EMPTY)>
     <!ATTLIST url              CDATA #REQUIRED>
 
-    <!ELEMENT project (annotation?,
+    <!ELEMENT project (annotation*,
                        project*)>
     <!ATTLIST project name        CDATA #REQUIRED>
     <!ATTLIST project path        CDATA #IMPLIED>