packagefs: Use '~' as pre-release separator

"[...]" turns out to be not so good an idea after all. In the shell it
would require escaping.

src/add-ons/kernel/file_systems/packagefs/Version.cpp

@@ -199,7 +199,7 @@ Version::ToString(char* buffer, size_t bufferSize) const
 
 	if (fPreRelease != NULL) {
 		size_t offset = std::min(bufferSize, size);
-		size += snprintf(buffer + offset, bufferSize - offset, "[%s]",
+		size += snprintf(buffer + offset, bufferSize - offset, "~%s",
 			fPreRelease);
 	}