Follow C-RW-VALUE in std::io::Cursor example

[mattfbacon]

rustc-dev-guide says to do this:

r? @steveklabnik

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then
please comment with @rustbot label +T-libs-api -T-libs to tag it
appropriately. If this PR contains changes to any unstable APIs please edit
the PR description to add a link to the relevant API Change
Proposal
or create one
if you haven't already. If you're unsure where your change falls no worries,
just leave it as is and the reviewer will take a look and make a decision to
forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing
  stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding
  permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[mattfbacon]

r? rust-lang/libs

[ChrisDenton]

@bors r+ rollup

[bors]

📌 Commit 39eb148 has been approved by ChrisDenton

It is now in the queue for this repository.