minor: fix bug in downcast_value! macro (T --> $T)

[alamb]

Which issue does this PR close?

N/A

Rationale for this change

There is a bug pointed out by @iajoiner#3484 (comment)

We don't see the problem because the only uses of the variant of the macro are all in generics that use T

When I remove that variant, you can see the locations

error: no rules expected the token `,`
--> datafusion/physical-expr/src/aggregate/approx_distinct.rs:252:58
|
252 | downcast_value!(values[0], GenericBinaryArray, T);
| ^ no rules expected this token in macro call
error: no rules expected the token `,`
--> datafusion/physical-expr/src/aggregate/approx_distinct.rs:268:58
|
268 | downcast_value!(values[0], GenericStringArray, T);
| ^ no rules expected this token in macro call
error: no rules expected the token `,`
--> datafusion/physical-expr/src/aggregate/approx_distinct.rs:284:82
|
284 | let array: &PrimitiveArray<T> = downcast_value!(values[0], PrimitiveArray, T);
| ^ no rules expected this token in macro call

What changes are included in this PR?

1. Change T to $T

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #3486 (adb4eb8) into master (9d028b3) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@ Coverage Diff @@## master #3486 +/- ##
=======================================
Coverage 85.75% 85.76% =======================================
Files 299 299 Lines 55282 55282 =======================================
+ Hits 47409 47410 +1 + Misses 7873 7872 -1 

Impacted Files	Coverage Δ
datafusion/common/src/lib.rs	0.00% <0.00%> (ø)
datafusion/expr/src/logical_plan/plan.rs	77.85% <0.00%> (+0.16%)	⬆️

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[ursabot]

Benchmark runs are scheduled for baseline = 06f01ea and contender = f52d8af. f52d8af is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java