Revise link to grammar syntax in grammar summary

The notation itself is not the "grammar production rules", so we don't
want to use that as the link text.  Let's instead point people to the
rule identifier by name.

Author: traviscross

src/grammar.md

@@ -1,7 +1,5 @@
 # Grammar summary
 
-The following is a summary of the [grammar production rules].
+The following is a summary of the grammar production rules. For details on the syntax of this grammar, see *[notation.grammar.syntax]*.
 
 {{ grammar-summary }}
-
-[grammar production rules]: notation.md