Fix small typo in tupleobject.h (#4801)

diff --git a/Include/tupleobject.h b/Include/tupleobject.h
index 98c2622..72a7d8d 100644
--- a/Include/tupleobject.h
+++ b/Include/tupleobject.h
@@ -10,7 +10,7 @@
 /*
 Another generally useful object type is a tuple of object pointers.
 For Python, this is an immutable type.  C code can change the tuple items
-(but not their number), and even use tuples are general-purpose arrays of
+(but not their number), and even use tuples as general-purpose arrays of
 object references, but in general only brand new tuples should be mutated,
 not ones that might already have been exposed to Python code.