Fix headers with custom IDs via the correct syntax

Was previously using <a name="..."> elements which just
add another anchor rather than changing the existing one.
They also use a deprecated HTML attribute and in some cases
broke the header display.

Fixes #1572.

Signed-off-by: Midnight Veil <midnightveil@fea.st>

content/client-server-api/modules/end_to_end_encryption.md

@@ -1699,7 +1699,7 @@ When a client is updating a Megolm session in its store, the client MUST ensure:
 
 {{% http-api spec="client-server" api="keys" %}}
 
-##### <a name="e2e-extensions-to-sync"> Extensions to /sync
+##### Extensions to /sync {#e2e-extensions-to-sync}
 
 This module adds an optional `device_lists` property to the [`/sync`](/client-server-api/#get_matrixclientv3sync)  response,
 as specified below. The server need only populate this property for an

content/server-server-api.md

@@ -877,8 +877,7 @@ on the resulting `m.room.member` event.
 If the joining server fails all conditions then a 403 `M_FORBIDDEN` error
 is used by the resident server.
 
-<a name="knocking-rooms">
-## Knocking upon a room
+## Knocking upon a room {#knocking-rooms}
 
 Rooms can permit knocking through the join rules, and if permitted this
 gives users a way to request to join (be invited) to the room. Users who