Fix variable name for valid times in lfp_band.py

[gshvarts]

This PR addresses #1481 .

Also wanted to confirm with someone that lines 326-346 look to be doing what they're supposed to be and don't have any typos, since the variable names are a little confusing for me to follow. Pasting them below too:

        valid_times = (
            IntervalList()
            & {
                "nwb_file_name": key["nwb_file_name"],
                "interval_list_name": interval_list_name,
            }
        ).fetch_interval()
        # the valid_times for this interval may be slightly beyond the valid
        # times for the lfp itself, so we have to intersect the two lists
        lfp_valid_times = (
            IntervalList()
            & {
                "nwb_file_name": key["nwb_file_name"],
                "interval_list_name": lfp_interval_list,
            }
        ).fetch_interval()

        lfp_band_valid_times = valid_times.intersect(
            lfp_valid_times,
            min_length=(LFPBandSelection & key).fetch1("min_interval_len"),
        )

Checklist:

• If this PR should be accompanied by a release, I have updated the CITATION.cff
• If this PR edits table definitions, I have included an alter snippet for release notes.
• If this PR makes changes to position, I ran the relevant tests locally.
• If this PR makes user-facing changes, I have added/edited docs/notebooks to reflect the changes
• I have updated the CHANGELOG.md with PR number and description.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.12%. Comparing base (32b6950) to head (653579b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1482   +/-   ##
=======================================
  Coverage   71.12%   71.12%           
=======================================
  Files         114      114           
  Lines       13306    13306           
=======================================
  Hits         9464     9464           
  Misses       3842     3842           

Flag	Coverage Δ
full-tests	71.12% <100.00%> (ø)

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