core-spec: Fix S_MatchData

Previously, it would substitute e for n without an e being around.
I clarify that by naming the scrutinee e.

docs/core-spec/OpSem.ott

@@ -66,9 +66,10 @@ S |- e --> e'
 S |- case e as n return t of </ alti // i /> --> case e' as n return t of </ alti // i />
 
 altj = K </ alphabb_kbb // bb /> </ xcc_tcc // cc /> -> u
+e = K </ t'aa // aa /> </ sbb // bb /> </ ecc // cc />
 u' = u[n |-> e] </ [alphabb_kbb |-> sbb] // bb /> </ [xcc_tcc |-> ecc] // cc />
 -------------------------------------------------------------- :: MatchData
-S |- case K </ t'aa // aa /> </ sbb // bb /> </ ecc // cc /> as n return t of </ alti // i /> --> u'
+S |- case e as n return t of </ alti // i /> --> u'
 
 altj = lit -> u
 ---------------------------------------------------------------- :: MatchLit