Update to Portenta X8 Image Building tutorial

[wgreycoon]

line 56 -- suggest deletion of recursive reference to directory 'lmp-manifest'. Can be fixed as suggested with this revision by replacing './lmp-manifest' with '.' in line 56, or alternately by removing line 55 ('cd lmp-manifest').

What This PR Changes

• fixes error caused by referring to a directory the user has already cd'd into

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