Fix typos “an”→“a”

library/stdarch/crates/std_detect/src/detect/arch/x86.rs

@@ -2,7 +2,7 @@
 //!
 //! The features are detected using the `detect_features` function below.
 //! This function uses the CPUID instruction to read the feature flags from the
-//! CPU and encodes them in an `usize` where each bit position represents
+//! CPU and encodes them in a `usize` where each bit position represents
 //! whether a feature is available (bit is set) or unavaiable (bit is cleared).
 //!
 //! The enum `Feature` is used to map bit positions to feature names, and the