[docs] Fix typo in QMessageAuthenticationCode::reset() documentation

Pefer → Prefer

Pick-to: 6.5
Change-Id: Ie9fd09452f5d99d799a045ee59fc11a2b9c5a599
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qcryptographichash.cpp

@@ -1284,7 +1284,7 @@ void QMessageAuthenticationCode::reset()
     use(mac);
     \endcode
 
-    Perfer to pass initial keys as the constructor argument:
+    Prefer to pass initial keys as the constructor argument:
 
     \code
     QMessageAuthenticationCode mac(method, key); // OK, optimal