Format example better

Signed-off-by: Jan Berktold <jberktold@roblox.com>

src/pulling_gauge.rs

@@ -14,8 +14,16 @@ use protobuf::RepeatedField;
 /// # Examples
 /// ```
 /// # use prometheus::{Registry, PullingGauge};
+/// # // We are stubbing out std::thread::available_parallelism since it's not available in the
+/// # // oldest Rust version that we support.
+/// # fn available_parallelism() -> f64 { 0.0 }
+///
 /// let registry = Registry::new();
-/// let gauge = PullingGauge::new("available_parallelism", "The available parallelism, usually the numbers of logical cores.", Box::new(|| std::thread::available_parallelism().unwrap().get() as f64)).unwrap();
+/// let gauge = PullingGauge::new(
+///     "available_parallelism",
+///     "The available parallelism, usually the numbers of logical cores.",
+///     Box::new(|| available_parallelism())
+/// ).unwrap();
 /// registry.register(Box::new(gauge));
 /// ```
 #[derive(Clone)]