Update BUILD.txt with correct Xcode spelling

Also update this file by replacing all XCode instances with Xcode.

BUILD.txt

@@ -47,9 +47,9 @@ may prefer to use the CMake GUI if unfamiliar with CMake.
 
 ### Mac OS X and Xcode ###
 
-NOTE: If you are using the new XCode that is downloaded from the AppStore
-and cmake complains about not finding XCode in /Developer, you have to run this
-command to update the XCode path:
+NOTE: If you are using the new Xcode that is downloaded from the AppStore
+and cmake complains about not finding Xcode in /Developer, you have to run this
+command to update the Xcode path:
 sudo /usr/bin/xcode-select -switch /Applications/Xcode.app/Contents/Developer
 
 To generate an Xcode project for building Polycode dependencies, perform