Update references to Mac OS X in comments.

OS X -> macOS.

Change-Id: I8a3f4a66b39272f62801ae1d36730ca4e0ef0741
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/72233
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index 9ad851f..fb92deb 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -66,7 +66,7 @@
 #define R_OK 4
 #endif
 
-// wordexp is a POSIX function that is only available on OSX and non-Android
+// wordexp is a POSIX function that is only available on macOS and non-Android
 // Linux platforms.
 #if defined(__APPLE__) || (defined(__linux__) && !defined(__ANDROID__))
 #define WORDEXP_AVAILABLE