uploader: remove inoperative `f`-string (#3311)
uploader: remove inoperative `f`-string (#3311)

Summary:
This `f`-string has no interpolations, so it *can* be a normal string,
and the Bazel target is marked `srcs_version = "PY2AND3"`, so it *must*
be a normal string.

Test Plan:
This module now parses under Python 2:

```
python2 -c '__import__("ast").parse(__import__("sys").stdin.read())' \
    <tensorboard/uploader/uploader_main.py
```

wchargin-branch: uploader-remove-fstring
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard