Fix comment for getPrefixed() in canonicalize_md.c

The fix for JDK-8235397 issue made a portion of a comment for getPrefixed() function in src/java.base/windows/native/libjava/canonicalize_md.c misleading.
The comment should be changed from

/* The appropriate location of getPrefixed() is io_util_md.c, but it is
   also used in a non-OpenJDK context within Oracle. There, canonicalize_md.c
   is already pulled in and compiled, so to avoid more complicated solutions
   we keep this method here.
 */

to:

/* The appropriate location of getPrefixed() is io_util_md.c
 */