fix signed/unsigned usage in BlockFilePath()

src/main.cpp

@@ -1993,7 +1993,7 @@ bool CheckDiskSpace(uint64 nAdditionalBytes)
 
 static filesystem::path BlockFilePath(unsigned int nFile)
 {
-    string strBlockFn = strprintf("blk%04d.dat", nFile);
+    string strBlockFn = strprintf("blk%04u.dat", nFile);
     return GetDataDir() / strBlockFn;
 }