Fix Subgraph README.md typo

[gwenall]

Why are these changes needed?

There is a typo int a shell example were there are 4 hyphens instead of 2 on the graph init parameters.
This commit is fixing this typo

Checks

This is a Readme typo.
The command has been tested locally.

[samlaf]

This looks reasonable to me. My guess is they wanted -- --from-contract ..., but not sure if the -- separation is needed for graph init.

[jianoaix]

This looks reasonable to me. My guess is they wanted -- --from-contract ..., but not sure if the -- separation is needed for graph init.

from-contract is a normal argument from graph, should not need the -- separation