Reference request for $ \pi_{1}(C,x) $ being generated by $ 2g $ elements for a smooth, connected, curve of genus $ g $

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I believe it is a result of Grothendieck that if $ x $ is a point of a smooth, connected curve $ C $ of genus $ g $ that $ \pi_{1}(C,x) $ is generated by $ 2g $ elements. Does anyone have a reference for this result?