Fix a spelling mistake

pep-0556.rst

@@ -49,7 +49,7 @@ There is a catch, though.  Over the course of reclaiming dead reference
 cycles (and any ancillary objects hanging at those cycles), the GC can
 execute arbitrary finalization code in the form of ``__del__`` methods
 and ``weakref`` callbacks.  Over the years, Python has been used for more
-and more sophisticated purposes, and it is increasinly common for
+and more sophisticated purposes, and it is increasingly common for
 finalization code to perform complex tasks, for example in distributed
 systems where loss of an object may require notifying other (logical
 or physical) nodes.