Indicate `None` is code-like in doc comment.

diff --git a/src/libstd/env.rs b/src/libstd/env.rs
index 40f6528f63efbdb577b995399d8fd00a97033837..f67f7e098d3450440416d03c02327889c5857bcb 100644 (file)
--- a/src/libstd/env.rs
+++ b/src/libstd/env.rs
@@ -181,7 +181,7 @@ fn _var(key: &OsStr) -> Result<String, VarError> {
 }
 
 /// Fetches the environment variable `key` from the current process, returning
-/// None if the variable isn't set.
+/// `None` if the variable isn't set.
 ///
 /// # Examples
 ///