Revert "build: correct the install rule for swiftdocs (#1137)" (#1144)

This reverts commit dba28a9.

Author: compnerd

CMakeLists.txt

@@ -281,7 +281,7 @@ if(ENABLE_SWIFT_NUMERICS)
 
     if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
       install(FILES $<TARGET_PROPERTY:${module},INTERFACE_INCLUDE_DIRECTORIES>/${module}.swiftdoc
-        DESTINATION lib/swift/${swift_os}/${module}.swiftdoc
+        DESTINATION lib/swift/${swift_os}/${module}.swiftmodule
         RENAME ${swift_arch}.swiftdoc)
       install(FILES $<TARGET_PROPERTY:${module},INTERFACE_INCLUDE_DIRECTORIES>/${module}.swiftmodule
         DESTINATION lib/swift/${swift_os}/${module}.swiftmodule