doc: Use "macOS" terminology consistently

One line in the documentation used the term macOS while the other six
used OSX. Be consistent and use the name current product brand of macOS.

Author: pmer

src/attributes.md

@@ -219,7 +219,7 @@ command line using `--cfg` (e.g. `rustc main.rs --cfg foo --cfg 'bar="baz"'`).
 Rust code then checks for their presence using the `#[cfg(...)]` attribute:
 
 ```
-// The function is only included in the build when compiling for OSX
+// The function is only included in the build when compiling for macOS
 #[cfg(target_os = "macos")]
 fn macos_only() {
   // ...

src/linkage.md

@@ -46,7 +46,7 @@ be ignored in favor of only building the artifacts specified by command line.
 * `--crate-type=cdylib`, `#[crate_type = "cdylib"]` - A dynamic system
   library will be produced.  This is used when compiling Rust code as
   a dynamic library to be loaded from another language.  This output type will
-  create `*.so` files on Linux, `*.dylib` files on OSX, and `*.dll` files on
+  create `*.so` files on Linux, `*.dylib` files on macOS, and `*.dll` files on
   Windows.
 
 * `--crate-type=rlib`, `#[crate_type = "rlib"]` - A "Rust library" file will be