GUIDE.md: remove `exit` command + minor grammar fixes
GUIDE.md: remove `exit` command + minor grammar fixes

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os97673/byebug,deivid-rodriguez/byebug,webdev1001/byebug,webdev1001/byebug,scalp42/byebug,yui-knk/byebug,guilherme/byebug,guilherme/byebug,deivid-rodriguez/byebug,yui-knk/byebug,icebreaker/byebug,k0kubun/byebug,icebreaker/byebug,yui-knk/byebug,deivid-rodriguez/byebug,k0kubun/byebug,k0kubun/byebug,os97673/byebug,scalp42/byebug,guilherme/byebug