Merge pull request #626 from jwilk/spelling-futures

[futures.shared_future] Remove duplicated word "shared"

Author: zygoloid

source/threads.tex

@@ -4364,7 +4364,7 @@
 
 \pnum
 \enternote Member functions of \tcode{shared_future} do not synchronize with themselves,
-but they synchronize with the shared shared state. \exitnote
+but they synchronize with the shared state. \exitnote
 
 \pnum
 The effect of calling any member function other than the destructor, the move-assignment