Improve clarity of 'content' error message for files_upload_v2

[eddyg]

Summary

Very simple PR to improve the clarity of the error message raised when 'content' is specified and it's not the correct type. (Feel free to edit as you see fit, or simply disregard if it's too minor.)

It is worth noting that the else case of this comparison (where neither content nor file_uploads is specified) is already handled here (for the async client) and here (regular).

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[seratch]

Thank you! Looks good to me.

[codecov]

Codecov Report

Merging #1278 (18496f7) into main (3fda85a) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1278      +/-   ##
==========================================
+ Coverage   85.63%   85.70%   +0.06%     
==========================================
  Files          84       84              
  Lines        7826     7826              
==========================================
+ Hits         6702     6707       +5     
+ Misses       1124     1119       -5     

Impacted Files	Coverage Δ
slack_sdk/socket_mode/builtin/internals.py	74.24% <0.00%> (+2.14%)	⬆️

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