spec: fix EBNF for slice syntax
spec: fix EBNF for slice syntax

The () parentheses grouped wrongly. Removed them completely in
favor of separate 2- and 3-index slice alternatives which is
clearer.

Fixes #14477.

Change-Id: I0b7521ac912130d9ea8740b8793b3b88e2609418
Reviewed-on: https://go-review.googlesource.com/19853
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

Samurais/go,Samurais/go,golang/go,golang/go,derekmarcotte/go,derekmarcotte/go,codestation/go,Cofyc/go,josharian/go,vsekhar/elastic-go,codestation/go,vanloswang/go,binarycrusader/go,golang/go,josharian/go,CAFxX/go,momchil-velikov/go,binarycrusader/go,CAFxX/go,vsekhar/elastic-go,mwhudson/go,miolini/go,goodlang/good,nawawi/go,theif519/go,polyverse-security/go,cloudflare/go,Samurais/go,akutz/go,akutz/go,ImJasonH/go,momchil-velikov/go,miolini/go,ganboing/go-esx,mwhudson/go,Cofyc/go,ganboing/go-esx,derekmarcotte/go,cloudflare/go,akutz/go,codestation/go,golang/go,vsekhar/elastic-go,cloudflare/go,suncycheng/go,vsekhar/elastic-go,akutz/go,ImJasonH/go,4ad/go,4ad/go,momchil-velikov/go,codestation/go,CAFxX/go,ganboing/go-esx,ImJasonH/go,vanloswang/go,ImJasonH/go,binarycrusader/go,ImJasonH/go,goodlang/good,josharian/go,polyverse-security/go,mwhudson/go,suncycheng/go,momchil-velikov/go,miolini/go,theif519/go,nawawi/go,ImJasonH/go,akutz/go,suncycheng/go,christopher-henderson/Go,polyverse-security/go,cloudflare/go,cloudflare/go,polyverse-security/go,vsekhar/elastic-go,mwhudson/go,CAFxX/go,ganboing/go-esx,Cofyc/go,cloudflare/go,ImJasonH/go,Cofyc/go,mwhudson/go,goodlang/good,4ad/go,golang/go,theif519/go,Samurais/go,linux-on-ibm-z/go,momchil-velikov/go,ganboing/go-esx,nawawi/go,ImJasonH/go,josharian/go,derekmarcotte/go,4ad/go,goodlang/good,cloudflare/go,theif519/go,linux-on-ibm-z/go,linux-on-ibm-z/go,akutz/go,Samurais/go,miolini/go,polyverse-security/go,momchil-velikov/go,golang/go,akutz/go,vsekhar/elastic-go,linux-on-ibm-z/go,polyverse-security/go,suncycheng/go,theif519/go,theif519/go,josharian/go,nawawi/go,ganboing/go-esx,vanloswang/go,4ad/go,linux-on-ibm-z/go,Cofyc/go,CAFxX/go,derekmarcotte/go,goodlang/good,CAFxX/go,nawawi/go,Cofyc/go,vanloswang/go,nawawi/go,vsekhar/elastic-go,ganboing/go-esx,mwhudson/go,Samurais/go,golang/go,Samurais/go,josharian/go,momchil-velikov/go,derekmarcotte/go,suncycheng/go,mwhudson/go,mwhudson/go,theif519/go,vanloswang/go,binarycrusader/go,ganboing/go-esx,codestation/go,Samurais/go,binarycrusader/go,CAFxX/go,codestation/go,nawawi/go,goodlang/good,linux-on-ibm-z/go,binarycrusader/go,golang/go,miolini/go,4ad/go,miolini/go,cloudflare/go,Cofyc/go,linux-on-ibm-z/go,4ad/go,vsekhar/elastic-go,goodlang/good,binarycrusader/go,nawawi/go,codestation/go,vanloswang/go,vanloswang/go,binarycrusader/go,codestation/go,derekmarcotte/go,derekmarcotte/go,polyverse-security/go,goodlang/good,josharian/go,polyverse-security/go,suncycheng/go,suncycheng/go,theif519/go,suncycheng/go,josharian/go,akutz/go,CAFxX/go,linux-on-ibm-z/go,4ad/go,momchil-velikov/go,vanloswang/go,miolini/go,Cofyc/go,miolini/go