cmake: make formatting of install statements follow one pattern

Change-Id: Ib7d571af629b0af78397a411b0cca409a6d218d8
diff --git a/docs/doxygen/CMakeLists.txt b/docs/doxygen/CMakeLists.txt
index 6ade92c..ce64cec 100644
--- a/docs/doxygen/CMakeLists.txt
+++ b/docs/doxygen/CMakeLists.txt
@@ -50,4 +50,8 @@
 
 add_custom_target(doxygen_target ALL DEPENDS ${BUILT_DIRS})
 
-install(DIRECTORY ${BUILT_DIRS} DESTINATION ${GR_PKG_DOC_DIR})
+install(
+    DIRECTORY
+    ${BUILT_DIRS}
+    DESTINATION ${GR_PKG_DOC_DIR}
+)