typo in io/Cursor.h

Summary: Closes https://github.com/facebook/folly/pull/721

Differential Revision: D6558163

Pulled By: yfeldblum

fbshipit-source-id: ed03aa92b54c61f9980a4759da29910aa77d5db6

folly/io/Cursor.h

@@ -39,7 +39,7 @@
  *
  * RWPrivateCursor - Read-write access, assumes private access to IOBuf chain
  * RWUnshareCursor - Read-write access, calls unshare on write (COW)
- * Appender        - Write access, assumes private access to IOBuf chian
+ * Appender        - Write access, assumes private access to IOBuf chain
  *
  * Note that RW cursors write in the preallocated part of buffers (that is,
  * between the buffer's data() and tail()), while Appenders append to the end