Fix typo in include_directories documentation (#12020)

A search-and-replace in commit 8d32d229 (make commands lower case by
default, 2007-10-10) accidentally changed the variable reference
CMAKE_INCLUDE_DIRECTORIES_BEFORE to CMAKE_include_directories_BEFORE.
Change it back.

Source/cmIncludeDirectoryCommand.h

@@ -61,7 +61,7 @@ public:
       "Add the given directories to those searched by the compiler for "
       "include files. By default the directories are appended onto "
       "the current list of directories. This default behavior can be "
-      "changed by setting CMAKE_include_directories_BEFORE to ON. "
+      "changed by setting CMAKE_INCLUDE_DIRECTORIES_BEFORE to ON. "
       "By using BEFORE or AFTER you can select between appending and "
       "prepending, independent from the default. "
       "If the SYSTEM option is given the compiler will be told that the "