Typo fixes

Doc/lib/libcollections.tex

@@ -279,7 +279,7 @@ a sequence of key-value pairs into a dictionary of lists:
 When each key is encountered for the first time, it is not already in the
 mapping; so an entry is automatically created using the
 \member{default_factory} function which returns an empty \class{list}.  The
-\method{list.append()} operation then attaches the value the new list.  When
+\method{list.append()} operation then attaches the value to the new list.  When
 keys are encountered again, the look-up proceeds normally (returning the list
 for that key) and the \method{list.append()} operation adds another value to
 the list. This technique is simpler and faster than an equivalent technique
@@ -308,7 +308,7 @@ languages):
 [('i', 4), ('p', 2), ('s', 4), ('m', 1)]
 \end{verbatim}
 
-When a letter in first encountered, it is missing from the mapping, so the
+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
 letter. This technique makes counting simpler and faster than an equivalent