Typo fix

Doc/lib/libcollections.tex

@@ -310,7 +310,7 @@ languages):
 
 When a letter is first encountered, it is missing from the mapping, so the
 \member{default_factory} function calls \function{int()} to supply a default
-count of zero.  The increment operation then builds of the count for each
+count of zero.  The increment operation then builds up the count for each
 letter. This technique makes counting simpler and faster than an equivalent
 technique using \method{dict.get()}: