Update Xcode version in the README

Author: radazzouz

README.md

@@ -95,7 +95,7 @@ If you want to work on PSPDFKitSwift, perform the following steps:
 * Clone `git@github.com:PSPDFKit/PSPDFKitSwift.git`
 * Copy `PSPDFKit.framework` into `PSPDFKitSwift/Frameworks`
 * Copy `PSPDFKitUI.framework` into `PSPDFKitSwift/Frameworks` (iOS only)
-* Open `PSPDFKitSwift.xcodeproj` in Xcode >= 10
+* Open `PSPDFKitSwift.xcodeproj` in Xcode >= 10.1
 
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