[flang] Fix typo in FlangConfig.cmake.in.

`find_package(Flang)` does not work as there is a missing `@` in the
FlangConfig.cmake.in file. This patch fixes the issue.

Reviewed By: thopre

Differential Revision: https://reviews.llvm.org/D96484

GitOrigin-RevId: 204360fd71d605d04cbe6e6efa3c2a898912c326
diff --git a/cmake/modules/FlangConfig.cmake.in b/cmake/modules/FlangConfig.cmake.in
index 3540e2d..5de85dc 100644
--- a/cmake/modules/FlangConfig.cmake.in
+++ b/cmake/modules/FlangConfig.cmake.in
@@ -6,7 +6,7 @@
              HINTS "@FLANG_CONFIG_LLVM_CMAKE_DIR@")
 
 set(FLANG_EXPORTED_TARGETS "@FLANG_EXPORTS@")
-set(FLANG_CMAKE_DIR "FLANG_CONFIG_CMAKE_DIR@")
+set(FLANG_CMAKE_DIR "@FLANG_CONFIG_CMAKE_DIR@")
 set(FLANG_INCLUDE_DIRS "@FLANG_CONFIG_INCLUDE_DIRS@")
 
 # Provide all our library targets to users.