Update reference/src/layout/enums.md

Co-Authored-By: Ralf Jung <[email protected]>

Author: gnzlbg

reference/src/layout/enums.md

@@ -322,7 +322,7 @@ as a niche.
 [`NonZeroU8`]: https://doc.rust-lang.org/std/num/struct.NonZeroU8.html
 
 The niche values must be disjoint from the values allowed by the validity
-invariant. The validity invariant is, as of this writing, is the current active
+invariant. The validity invariant is, as of this writing, the current active
 discussion topic in the unsafe code guidelines process. [rust-lang/rust#60300]
 specifies that the following types have at least one niche (the all-zeros
 bit-pattern):