Actually change literal delimiters (#366)

This is a follow-up to #359, in which I changed the docs and the escape sequence productions, only to forget to edit the delimiters themselves...

Author: stasm

spec/message.abnf

@@ -31,7 +31,7 @@ text-char = %x0-5B         ; omit \
           / %x7E-D7FF      ; omit surrogates
           / %xE000-10FFFF
 
-literal = "(" *(literal-char / literal-escape) ")"
+literal = "|" *(literal-char / literal-escape) "|"
 literal-char = %x0-5B         ; omit \
              / %x5D-7B        ; omit |
              / %x7D-D7FF      ; omit surrogates

spec/syntax.md

@@ -445,7 +445,7 @@ text-char = %x0-5B         ; omit \
 ```
 
 ```abnf
-literal = "(" *(literal-char / literal-escape) ")"
+literal = "|" *(literal-char / literal-escape) "|"
 literal-char = %x0-5B         ; omit \
              / %x5D-7B        ; omit |
              / %x7D-D7FF      ; omit surrogates