Fix typo in default_allocator (#187)

Author: MambaWong

include/foonathan/memory/default_allocator.hpp

@@ -25,7 +25,7 @@ namespace foonathan
         /// Arena allocators like \ref memory_stack or \ref memory_pool allocate memory by subdividing a huge block.
         /// They get a \concept{concept_blockallocator,BlockAllocator} that will be used for their internal allocation,
         /// this type is the default value.
-        /// \requiredbe Its type can be changed via the CMake option \c FOONATHAN_MEMORY_DEFAULT_ALLCOATOR,
+        /// \requiredbe Its type can be changed via the CMake option \c FOONATHAN_MEMORY_DEFAULT_ALLOCATOR,
         /// but it must be one of the following: \ref heap_allocator, \ref new_allocator, \ref malloc_allocator, \ref static_allocator, \ref virtual_memory_allocator.
         /// \defaultbe The default is \ref heap_allocator.
         /// \ingroup allocator