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.

src/libcore/default.rs

@@ -54,7 +54,7 @@
 ///
 /// ## How can I implement `Default`?
 ///
-/// Provides an implementation for the `default()` method that returns the value of
+/// Provide an implementation for the `default()` method that returns the value of
 /// your type that should be the default:
 ///
 /// ```