Change effective dart rule about line length (#6404)

Fixes #6142

Author: MaryaBelanger

src/content/effective-dart/style.md

@@ -454,8 +454,8 @@ formatting the code by hand and trying to make it more readable. Think of
 `dart format` as a partnership where you work together, sometimes iteratively, 
 to produce beautiful code.
 
-
-### AVOID lines longer than 80 characters
+<a id="avoid-lines-longer-than-80-characters"></a>
+### PREFER lines 80 characters or fewer
 
 {% render 'linter-rule-mention.md', rules:'lines_longer_than_80_chars' %}
 
@@ -469,7 +469,8 @@ compact. The main offender is usually `VeryLongCamelCaseClassNames`. Ask
 yourself, "Does each word in that type name tell me something critical or
 prevent a name collision?" If not, consider omitting it.
 
-Note that `dart format` does 99% of this for you, but the last 1% is you. 
+Note that `dart format` defaults to 80 characters or fewer, though you can
+[configure][] the default. 
 It does not split long string literals to fit in 80 columns, 
 so you have to do that manually.
 
@@ -481,6 +482,8 @@ an import or export), it may remain whole even if it causes the line to go over
 because newlines are significant inside the string and splitting the lines into
 shorter ones can alter the program.
 
+[configure]: /tools/dart-format#configuring-formatter-page-width
+
 <a id="do-use-curly-braces-for-all-flow-control-structures"></a>
 ### DO use curly braces for all flow control statements