Rollup merge of rust-lang#34610 - wuranbo:patch-2, r=steveklabnik

doc: make the conditional-compilation example work

If not, the error `does not have these features: foo` confused.
r? @steveklabnik

src/doc/book/conditional-compilation.md

@@ -41,8 +41,9 @@ they get set in the [`[features]` section][features] of your `Cargo.toml`:
 # no features by default
 default = []
 
-# The “secure-password” feature depends on the bcrypt package.
-secure-password = ["bcrypt"]
+# Add feature "foo" here, then you can use it. 
+# Our "foo" feature depends on nothing else.
+foo = []
 ```
 
 When you do this, Cargo passes along a flag to `rustc`: