Tweak PDU diagram demonstrating prev_events. (#3340)

This tweaks the DAG to be simpler, with two linear event chains E4 -> E3
-> E2 -> E1 and E6 -> E5 -> E2 -> E1. The extremities of the DAG are now
the first and only point in the DAG where multiple event parents occur.
Since the point of the diagram is to demonstrate this very situation,
it's better didactically if there is only one such situation in the
diagram.

Author: dkasak

changelogs/server_server/newsfragments/3340.clarification

@@ -0,0 +1 @@
+Tweak the example PDU diagram to better demonstrate situations with multiple `prev_events`.

content/server-server-api.md

@@ -333,15 +333,16 @@ after all other known events.
 
 For example, consider a room whose events form the DAG shown below. A
 server creating a new event in this room should populate the new event's
-`prev_events` field with `E4` and `E5`, since neither event yet has a
-child:
+`prev_events` field with both `E4` and `E6`, since neither event yet has
+a child:
 
     E1
     ^
     |
-    +-> E2 <-+
-    |        |
-    E3       E5
+    E2 <--- E5
+    ^       ^
+    |       |
+    E3      E6
     ^
     |
     E4