Update documentation of binary size tool.

The '--report-file' argument of html_report is already removed and replaced with
positional arguments, update the documentation accordingly.

Bug: None
Change-Id: I2ee343faffb55e4be8c5bf7b4d51503fb4893781
Reviewed-on: https://chromium-review.googlesource.com/c/1390022Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619402}

tools/binary_size/README.md

@@ -242,13 +242,13 @@ Example Usage:
 
 ``` bash
 # Creates the data file ./report.ndjson, generated based on ./chrome.size
-tools/binary_size/supersize html_report chrome.size --report-file report.ndjson -v
+tools/binary_size/supersize html_report chrome.size report.ndjson -v
 
 # Includes every symbol in the data file, although it will take longer to load.
-tools/binary_size/supersize html_report chrome.size --report-file report.ndjson --all-symbols
+tools/binary_size/supersize html_report chrome.size report.ndjson --all-symbols
 
 # Create a data file showing a diff between two .size files.
-tools/binary_size/supersize html_report after.size --diff-with before.size --report-file report.ndjson
+tools/binary_size/supersize html_report after.size --diff-with before.size report.ndjson
 ```
 
 ### Usage: start_server