parse.y: fixed bison 3.0 warning

YACC  src/parse.y -> build/host/src/y.tab.c
src/parse.y:936.1-12: warning: deprecated directive, use ‘%pure-parser’
[-Wdeprecated]
%pure_parser

src/parse.y

@@ -933,7 +933,7 @@ heredoc_end(parser_state *p)
 
 %}
 
-%pure_parser
+%pure-parser
 %parse-param {parser_state *p}
 %lex-param {parser_state *p}