Update constraints.md (flutter#10070)

Updated the values of constraints for Widget.

_Description of what this PR is changing or adding, and why:_

_Issues fixed by this PR (if any):_

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Co-authored-by: Brett Morgan <brettmorgan@google.com>

src/ui/layout/constraints.md

@@ -69,8 +69,8 @@ The negotiation goes something like this:
 
 **Widget**: "Hey parent, what are my constraints?"
 
-**Parent**: "You must be from `80` to `300` pixels wide,
-   and `30` to `85` tall."
+**Parent**: "You must be from `0` to `300` pixels wide,
+   and `0` to `85` tall."
 
 **Widget**: "Hmmm, since I want to have `5` pixels of padding,
    then my children can have at most `290` pixels of width