fix misspelling lpm -> lmp in portenta x8 content.md

[e1ke]

What This PR Changes

I followed the tutorial https://docs.arduino.cc/tutorials/portenta-x8/image-flashing/#arduinos-download-repository and noticed the misspelling of lmp-manifest repository

I changed

• lpm to lmp in the image subtitle
• lpm to lmp in the image link to assets/lmp-manifest-overview.png
• rename lpm-manifest-overview.png to lmp-manifest-overview.png

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