minor: rename Ua to A and Ub to B in contact.seq

much easier to read this way.
suggested by @hpk in #78

images/contact.seq

@@ -2,23 +2,23 @@
 # I used https://bramp.github.io/js-sequence-diagrams/
 # to render it to svg and then save it in images folder
 
-participant Alice as Ua
-participant Bob as Ub
+participant Alice as A
+participant Bob as B
 
-Ua --> Ub: 1.a) bootstrap code
-Note over Ua: 1.b) track bootstrap per INVITENUMBER
-Note over Ub: 2.a) check for exising key
-Ub ->> Ua: 2.b) vc-request message with INVITENUMBER
-Note over Ua: 3.a) look up bootstrap by INVITENUMBER
-Note over Ua: 3.b) abort if invite expired
-Note over Ua: 3.c) process AC header
-Ua -> Ub: 3.d) vc-auth-required message with AC header
-Note over Ub: 4.a) abort if key does not match FP from bootstrap
-Ub -> Ua: 4.b) vc-request-with-auth with Bob_FP and AUTH
-Note over Ua: 5.a) verify AUTH and key
-Note over Ua: 5.b) on failure alert user and abort
-Note over Ua: 6.a) signal success to user
-Ua -> Ub: 6.b) vc-contact-confirm message
-Note over Ua: 6.c) clear bootstrap data for INVITECODE
-Note over Ub: 7. signal success to user
+A --> B: 1.a) bootstrap code
+Note over A: 1.b) track bootstrap per INVITENUMBER
+Note over B: 2.a) check for exising key
+B ->> A: 2.b) vc-request message with INVITENUMBER
+Note over A: 3.a) look up bootstrap by INVITENUMBER
+Note over A: 3.b) abort if invite expired
+Note over A: 3.c) process AC header
+A -> B: 3.d) vc-auth-required message with AC header
+Note over B: 4.a) abort if key does not match FP from bootstrap
+B -> A: 4.b) vc-request-with-auth with Bob_FP and AUTH
+Note over A: 5.a) verify AUTH and key
+Note over A: 5.b) on failure alert user and abort
+Note over A: 6.a) signal success to user
+A -> B: 6.b) vc-contact-confirm message
+Note over A: 6.c) clear bootstrap data for INVITECODE
+Note over B: 7. signal success to user