mac: fix typo

I happened to notice this typo while investigating an unrelated error.

Change-Id: Ic4148929b3654a76cfd78f86a61a559b6c36a100
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1866976
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Joshua Peraza <jperaza@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707336}

build/config/mac/BUILD.gn

@@ -77,7 +77,7 @@ config("runtime_library") {
   # macros that collide with common names, like 'check', 'require', and
   # 'verify'.
   # http://opensource.apple.com/source/CarbonHeaders/CarbonHeaders-18.1/AssertMacros.h
-  defines = [ "__ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORE=0" ]
+  defines = [ "__ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORES=0" ]
 }
 
 # On Mac, this is used for everything except static libraries.