Fix typo in Default trait docs: Provides -> Provide
Fix typo in Default trait docs: Provides -> Provide

An earlier commit (99ed06e) accidentally changed this paragraph from the
original, imperative "Provide" to the present tense "Provides". The
latter is indeed the standard for Rustdoc comments relating to a
function or method, but this snippet is introducing the Default trait in
general terms and not talking about any particular function. I believe
this change was likely made in error and should be reverted.

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