fix bikeshed error (disambiguate #blockwidth reference) (#566)

Author: ylafon

index.bs

@@ -3529,7 +3529,7 @@ Sometimes monkey patching is unavoidable (e.g., early in the design phase of a n
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-    In terms of CSS2.1 block-level formatting [[CSS2]], the rules for “over-constrained” computations in [[CSS2#blockwidth]] are ignored in favor of alignment as specified here and the used value of the margin properties are therefore not adjusted to correct for the over-constraint.
+    In terms of CSS2.1 block-level formatting [[CSS2]], the rules for “over-constrained” computations in [[CSS2/#blockwidth]] are ignored in favor of alignment as specified here and the used value of the margin properties are therefore not adjusted to correct for the over-constraint.
 
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