Rollup merge of #40175 - d-e-s-o:fix-inconsistency-in-guessing-game-readme, r=stevekl...

doc: fix inconsistency in error output in guessing-game.md

The line '.expect("failed to read line");' is partly started with a
lower case 'f' and partly with an uppercase one, adding additional
spurious changes to otherwise clean diffs if each sample is
copy-and-pasted over the previous.
This change starts the string with an uppercase everywhere which is in
line with the style of the other strings.