Change fcm diff error message

[Pierre-siddall]

PR Summary

Sci/Tech Reviewer:
Code Reviewer: @t00sa

Fixes the misleading error message in umdp3 fixer which references fcm.

closes #189

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[t00sa]

See comment in line

[t00sa]

Almost but not quite: if you look at the command above, it's running plain diff rather than git diff

Suggested change

	"Ran git diff command and changes were made by "
	"Ran diff command and changes were made by "