Updated libpng url in README (#77) #44

Job	Run time
Julia 1 - ubuntu-latest - x64 - push	4m 58s
Julia 1 - ubuntu-latest - x86 - push	5m 53s
Julia 1 - macOS-latest - aarch64 - push	3m 18s
Julia 1 - macOS-13 - x64 - push	5m 23s
Julia 1 - windows-latest - x64 - push	6m 7s
Julia 1 - windows-latest - x86 - push	7m 57s
Julia lts - ubuntu-latest - x64 - push	2m 43s
Julia lts - ubuntu-latest - x86 - push	3m 6s
Julia lts - macOS-latest - aarch64 - push	2m 1s
Julia lts - macOS-13 - x64 - push	3m 15s
Julia lts - windows-latest - x64 - push	3m 30s
Julia lts - windows-latest - x86 - push	4m 32s
Julia 1.6 - ubuntu-latest - x64 - push	2m 50s
Julia 1.6 - ubuntu-latest - x86 - push	3m 26s
Julia 1.6 - macOS-13 - x64 - push	4m 16s
Julia 1.6 - windows-latest - x64 - push	9m 53s
Julia 1.6 - windows-latest - x86 - push	11m 41s
Julia nightly - ubuntu-latest - x64 - push	4m 35s
	1h 29m 24s

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