fix small typos in std::convert documentation
fix small typos in std::convert documentation

Fix subject-verb agreement in copypasta: "`AsRef` dereference" to
"`AsRef` dereferences".

Formalize "eg" to "e.g." Italicization of common Latin abbreviations
seems to be going out of style in written English, so I left it plain.

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