Merge pull request #3738 from stu-h/patch-1

Reword "`union`s" to "a `union`"

src/ch19-01-unsafe-rust.md

@@ -34,7 +34,7 @@ include the ability to:
 * Call an unsafe function or method
 * Access or modify a mutable static variable
 * Implement an unsafe trait
-* Access fields of `union`s
+* Access fields of a `union`
 
 It’s important to understand that `unsafe` doesn’t turn off the borrow checker
 or disable any other of Rust’s safety checks: if you use a reference in unsafe