typo in mtbl.pyx MTBL sorter docs

[reedjc]

Typo fix:

diff --git a/mtbl.pyx b/mtbl.pyx
index b15c853..82365c9 100644
--- a/mtbl.pyx
+++ b/mtbl.pyx
@@ -586,7 +586,7 @@ cdef class sorter(object):
 
     Keyword arguments:
     temp_dir -- temporary directory (default "/var/tmp")
-    max_memory -- maxmimum amount of memory for in-memory sorting in bytes (default 1 GB)
+    max_memory -- maximum amount of memory for in-memory sorting in bytes (default 1 GB)
     """
     cdef mtbl_sorter *_instance
     cdef _lock