Update documentation for requiring the 10.15 SDK.

BUG=1023913

Change-Id: I6c581846479edd6f7938ab039bef38f3c38eb251
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036492
Reviewed-by: Elly Fong-Jones <[email protected]>
Commit-Queue: Avi Drissman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#742296}
diff --git a/docs/mac_build_instructions.md b/docs/mac_build_instructions.md
index 71a926b0..5a7665e3 100644
--- a/docs/mac_build_instructions.md
+++ b/docs/mac_build_instructions.md
@@ -12,16 +12,16 @@
 
 ## System requirements
 
-*   A 64-bit Mac running 10.13+.
-*   [Xcode](https://developer.apple.com/xcode) 10+
-*   The OS X 10.14 SDK. Run
+*   A 64-bit Mac running 10.14+.
+*   [Xcode](https://developer.apple.com/xcode) 11+
+*   The OS X 10.15 SDK. Run
 
     ```shell
     $ ls `xcode-select -p`/Platforms/MacOSX.platform/Developer/SDKs
     ```
 
     to check whether you have it.  Building with a newer SDK works too, but
-    the releases currently use the 10.14 SDK.
+    the releases currently use the 10.15 SDK.
 
 ## Install `depot_tools`