Fix MacOS cmake build instructions (KhronosGroup#1363)

Co-authored-by: Wasim Abbas <abbas.wasim@gmail.com>

Author: wasimabbas-arm

docs/build.adoc

@@ -271,15 +271,15 @@ source /PATH/TO/VULKAN/SDK/setup-env.sh
 `Step 1.` The following command will generate the project
 
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-cmake -G Xcode -Bbuild/mac-xcode -DCMAKE_BUILD_TYPE=Release
+cmake -G Xcode -Bbuild/mac-xcode -DCMAKE_BUILD_TYPE=Release -DCMAKE_OSX_SYSROOT=macosx
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 Open the *vulkan_samples* Xcode project inside build/mac-xcode and build with command-B.  To run Vulkan Samples, use Xcode's edit-scheme selection and set the arguments to --help. Click the "Play" button and you should see the help output in the terminal. For convenience, the default setting is to run the hello_triangle sample; just edit that to your desired sample to run.
 
 Alternatively, for command line builds use the steps below:
 
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-cmake -Bbuild/mac -DCMAKE_BUILD_TYPE=Release
+cmake -Bbuild/mac -DCMAKE_BUILD_TYPE=Release -DCMAKE_OSX_SYSROOT=macosx
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 `Step 2.` Build the project