r.category manual: minor addition

be clear that the rules file is without header

raster/r.category/r.category.html

@@ -37,7 +37,7 @@ another program.
 <h3>Input from a file</h3>
 
 The <b>rules</b> option allows the user to assign category labels from values
-found in a file. The label can refer to a single category, range of
+found in a file (without header). The label can refer to a single category, range of
 categories, floating point value, or a range of floating point values.
 The format is given as follows (when separator is set to colon; no white space
 must be used after the separator):