Auto merge of #47036 - QuietMisdreavus:i-like-big-chars, r=frewsxcv
Auto merge of #47036 - QuietMisdreavus:i-like-big-chars, r=frewsxcv

update char_indices example to highlight big chars

There was a comment today in IRC where someone thought `char_indices()` and `chars().enumerate()` were equivalent, so i wanted to put an example in the docs where that wasn't true.

r? @rust-lang/docs

graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust