README: account for funky xcode paths

Signed-off-by: Jason A. Donenfeld <[email protected]>

Author: zx2c4

README.md

@@ -54,7 +54,7 @@ $ open WireGuard.xcodeproj
      the "External Build Tool Configuration":
 
      ```
-     $BUILD_DIR/../../SourcePackages/checkouts/wireguard-apple/Sources/WireGuardKitGo
+     ${BUILD_DIR%Build/*}SourcePackages/checkouts/wireguard-apple/Sources/WireGuardKitGo
      ```
      
    - Switch to "Build Settings" and find `SDKROOT`.