Update ToBase64String

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Removes the TPixel type parameter from the ToBase64String(this Image source, IImageFormat format) method as it was unnecessary. Also fleshed out the intellisense documentation to correctly show the example output.

[tocsoft]

LGTM 👍

[codecov]

Codecov Report

Merging #1175 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1175   +/-   ##
=======================================
  Coverage   82.40%   82.40%           
=======================================
  Files         684      684           
  Lines       29519    29519           
  Branches     3328     3328           
=======================================
  Hits        24324    24324           
  Misses       4507     4507           
  Partials      688      688           

Flag	Coverage Δ
#unittests	82.40% <ø> (ø)

Impacted Files	Coverage Δ
src/ImageSharp/ImageExtensions.cs	68.42% <ø> (ø)

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