src/ietf/rfc5234-abnf.pegjs: fix alternation rule
src/ietf/rfc5234-abnf.pegjs: fix alternation rule

From http://tools.ietf.org/html/rfc5234#page-11

         alternation    =  concatenation
                           *(*c-wsp "/" *c-wsp concatenation)

for-GET/core-pegjs