Use code formatting for code-relevant values in std::fmt docs
Use code formatting for code-relevant values in std::fmt docs

Especially when documenting the use of `0`, since zero looks very
similar to `O` in fonts not meant for displaying code.

Other literal characters, traits, etc should also use code formatting.
This change makes this documentation more internally consistent.

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