FIX: fix typo in piece.h

paddle/string/piece.h

@@ -35,7 +35,7 @@ public:
 
   // We provide non-explicit singleton constructors so users can
   // pass in a "const char*" or a "string" wherever a "Piece"
-  // is expected.  These contructors ensure that if data_ is NULL,
+  // is expected.  These constructors ensure that if data_ is NULL,
   // size_ is 0.
   Piece();
   Piece(const char* d, size_t n);