How do I get Path Auto to display one value in the URL structure when node:field has multiple values?
For multiple cardinality fields you can use a pattern like [node:field-name:1] (note the use of hyphens instead of underscores). 

This is an example of the tokens available for a field called field_profile_categories with multiple values:



I think in your case the pattern should be:

[node:field-restaurant-category:1]


Although the comments in the token table suggest that deltas start at 0, so you might need to use [node:field-restaurant-category:0] if you want the first item to be used.