style: fix typo in code comment (#532)

src/header/value.rs

@@ -79,7 +79,7 @@ impl HeaderValue {
     /// assert_eq!(val, "hello");
     /// ```
     #[inline]
-    #[allow(unconditional_panic)] // required for the panic circumventon
+    #[allow(unconditional_panic)] // required for the panic circumvention
     pub const fn from_static(src: &'static str) -> HeaderValue {
         let bytes = src.as_bytes();
         let mut i = 0;