include/linux/generic-radix-tree.h: fix kerneldoc comment

The DOC comment block section in include/linux/generic-radix-tree.h
contained a spurious colon, causing this warning in the documentation
build:

  include/linux/generic-radix-tree.h:1: warning: no structured comments found

Remove the colon and make the docs build happy.

Link: http://lkml.kernel.org/r/20190524141933.74ae9050@lwn.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/generic-radix-tree.h

@@ -2,7 +2,7 @@
 #define _LINUX_GENERIC_RADIX_TREE_H
 
 /**
- * DOC: Generic radix trees/sparse arrays:
+ * DOC: Generic radix trees/sparse arrays
  *
  * Very simple and minimalistic, supporting arbitrary size entries up to
  * PAGE_SIZE.