Fix name capitalization

[karlhorky]

Hi, thanks for Oxipng!

I saw further down in the readme that Oxipng was being used in the middle of a sentence with a capital O, so I adjusted the other lowercase usages (other than the usages in backticks, which may instead refer to the command line program itself) from "oxipng" to also use "Oxipng"

[ace-dent]

Need to check in with @shssoichiro what the naming intention is! :-)

[AlexTMjugador]

Oddly enough, I tend to capitalize OxiPNG like that, not as Oxipng or oxipng... 😂

[andrews05]

@AlexTMjugador Personally, I think that makes it look too similar to OptiPNG 😆

[ace-dent]

Not sure if it's open to voting(!?)... but my preference is oxipng and capitalised at the start of a sentence.

[karlhorky]

oxipng and capitalised at the start of a sentence.

Usually mixing like this will lead to style confusion - often it's better to have a single, strong capitalization style for a brand name and stick with it (regardless of where the word is used in the sentence).

However, that's not to say that the brand name for the project / ecosystem can't be styled differently than the brand name for the command line interface, for example:

1. Oxipng (capital O, everything else lowercase) for the brand name for the project / ecosystem / anything else
2. oxipng (all lowercase, enclosed in backticks) for the brand name for the command line interface program