avutil/csp: Fix bogus doxy filename

Separate the blocks to make the grouping easier to grasp,
add the file properly to the group and fix the file description
incorrectly used as filename, fixing the following doxy warning:

  warning: the name 'Colorspace' supplied as the argument in
  the \file statement is not an input file

libavutil/csp.h

@@ -24,9 +24,14 @@
 #include "rational.h"
 
 /**
- * @file Colorspace value utility functions for libavutil.
+ * @file
+ * Colorspace value utility functions for libavutil.
+ * @ingroup lavu_math_csp
  * @author Ronald S. Bultje <rsbultje@gmail.com>
  * @author Leo Izen <leo.izen@gmail.com>
+ */
+
+/**
  * @defgroup lavu_math_csp Colorspace Utility
  * @ingroup lavu_math
  * @{