[nfc] Fix mistake in probe type grammar

Fix incorrect trailing comma when specifying a layer-associated probe
type.

Signed-off-by: Schuyler Eldridge <[email protected]>

revision-history.yaml

@@ -12,6 +12,7 @@ revisionHistory:
       - Add section "Circuit Components".
       - Reorganized statements section.
       - Rewrite of the Types section.
+      - Fix mistake in layer-associated probe grammar.
     abi:
       - Add ABI for public modules and filelist output.
       - Changed ABI for group and ref generated files.

spec.md

@@ -3512,7 +3512,7 @@ type_enum = "{|" , { type_enum_alt } , "|}" ;
 type_enum_alt = id, [ ":" , type_constable ] ;
 
 (* Probe Types *)
-type_probe = ( "Probe" | "RWProbe" ) , "<", type , [ "," , id , "," ] ">" ;
+type_probe = ( "Probe" | "RWProbe" ) , "<", type , [ "," , id ] ">" ;
 
 (* Primitive Operations *)
 primop_2expr     = primop_2expr_keyword , "(" , expr , "," , expr ")" ;