Fix typos “an”→“a” and a few different ones that appeared in the same search

library/std/src/keyword_docs.rs

@@ -2289,7 +2289,7 @@ mod dyn_keyword {}
 /// }
 ///
 /// let mut u = IntOrFloat { f: 1.0 };
-/// // Reading the fields of an union is always unsafe
+/// // Reading the fields of a union is always unsafe
 /// assert_eq!(unsafe { u.i }, 1065353216);
 /// // Updating through any of the field will modify all of them
 /// u.i = 1073741824;