Fix typo.

Doc/lib/libdecimal.tex

@@ -1095,7 +1095,7 @@ def cos(x):
     return +s
 
 def sin(x):
-    """Return the cosine of x as measured in radians.
+    """Return the sine of x as measured in radians.
 
     >>> print sin(Decimal('0.5'))
     0.4794255386042030002732879352