📚 doc: fix GFM table format

doc/spec.md

@@ -278,52 +278,30 @@ Each lexer has its own mode. The mode of a lexer is either `A` or `S`. The initi
 
 The complete conversion table between instructions and their Watson Representations are as follows:
 
-+-----------+--------------+--------------+
-|Instruction\mode|A        |S        |
-+-----------+--------------+--------------+
+
+| insn\mode |A             |S             |
+|-----------|--------------|--------------|
 |Inew       |B             |S             |
-+-----------+--------------+--------------+
 |Iinc       |u             |h             |
-+-----------+--------------+--------------+
 |Ishl       |b             |a             |
-+-----------+--------------+--------------+
 |Iadd       |a             |k             |
-+-----------+--------------+--------------+
 |Ineg       |A             |r             |
-+-----------+--------------+--------------+
 |Isht       |e             |A             |
-+-----------+--------------+--------------+
 |Itof       |i             |z             |
-+-----------+--------------+--------------+
 |Itou       |'             |i             |
-+-----------+--------------+--------------+
 |Finf       |q             |m             |
-+-----------+--------------+--------------+
 |Fnan       |t             |b             |
-+-----------+--------------+--------------+
 |Fneg       |p             |u             |
-+-----------+--------------+--------------+
 |Snew       |?             |$             |
-+-----------+--------------+--------------+
 |Sadd       |!             |-             |
-+-----------+--------------+--------------+
 |Onew       |~             |+             |
-+-----------+--------------+--------------+
 |Oadd       |M             |g             |
-+-----------+--------------+--------------+
 |Anew       |@             |v             |
-+-----------+--------------+--------------+
 |Aadd       |s             |?             |
-+-----------+--------------+--------------+
 |Bnew       |z             |^             |
-+-----------+--------------+--------------+
 |Bneg       |o             |!             |
-+-----------+--------------+--------------+
 |Nnew       |.             |y             |
-+-----------+--------------+--------------+
 |Gdup       |E             |/             |
-+-----------+--------------+--------------+
 |Gpop       |#             |e             |
-+-----------+--------------+--------------+
 |Gswp       |%             |:             |
-+-----------+--------------+--------------+
+