doc: remove block_dec_error productions from grammar.txt

[crusso]

::= '{' <list_er(, <block_dec_error>, ';')> '}'

<block_dec_error> ::= '=' '=' ';' <list(<exp_field>, ';')> <with_exp_fields>

Should be simplified to:

::= '{' <list(, ';')> '}'

(using grammar.sed)