Span: Fixed comment referencing std::span as_writable_bytes() as as_mutable_bytes().

PiperOrigin-RevId: 605288522
Change-Id: Id16ad96b9323c71bd60801a6c8c359f7ff3d320e

absl/types/span.h

@@ -43,7 +43,7 @@
 //    * A read-only `absl::Span<const T>` can be implicitly constructed from an
 //      initializer list.
 //    * `absl::Span` has no `bytes()`, `size_bytes()`, `as_bytes()`, or
-//      `as_mutable_bytes()` methods
+//      `as_writable_bytes()` methods
 //    * `absl::Span` has no static extent template parameter, nor constructors
 //      which exist only because of the static extent parameter.
 //    * `absl::Span` has an explicit mutable-reference constructor