Use match expression directly in guide.md

Use a match expression directly in the println statement, instead of creating a second variable.
auto merge of #18120 : jrincayc/rust/match_exp, r=thestinger

Use a match expression directly in the println statement, instead of creating a second variable.  It seems weird that the current guide.md complains about creating an extra variable, when the same feature could be demonstrated without creating the extra variable.