Fix format of manpage

Signed-off-by: Antonio Niño Díaz <[email protected]>

src/asm/rgbasm.5

@@ -50,7 +50,7 @@ syntax to do so is the following one:
 .Pp
 This works anywhere in the code except inside of strings. To split strings it is
 needed to use
-.It STRCAT
+.Sy STRCAT
 like this:
 .Pp
 .Bd -literal -offset indent