Change position guard from MustBeLessThanTo to MustBeLessThanOrEqualTo

[brianpopow]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

As the title says: The guard of the position was a bit too strict.

[codecov]

Codecov Report

Merging #1291 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1291   +/-   ##
=======================================
  Coverage   82.70%   82.70%           
=======================================
  Files         697      697           
  Lines       30677    30677           
  Branches     3470     3470           
=======================================
  Hits        25370    25370           
  Misses       4604     4604           
  Partials      703      703           

Flag	Coverage Δ
#unittests	82.70% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/ImageSharp/IO/BufferedReadStream.cs	92.42% <100.00%> (ø)

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