Fix typo in library documentation.

src/site/apt/library.apt

@@ -203,7 +203,7 @@ while reader.next():
   The command may however be re-written like this:
   
 +-------------------------------------
-A:MASK(Huang),B:MASK(Moments),3:COPY()
+A:MASK(Huang),B:MASK(Moments),C:COPY()
 +-------------------------------------
 
   With a channel order specification of <<<ABC>>>, that command will be
@@ -301,4 +301,4 @@ while batch.next():
         results.addValue("Mean", stats.mean)
 
     image.close()
-+-----------------------------------------------------------------------
++-----------------------------------------------------------------------