Rename notation to "grammar notation"

This is in preparation to rearrange notational conventions. This chapter
*only* deals with grammar, so let's be explicit about that.

Author: ehuss

src/SUMMARY.md

@@ -2,7 +2,7 @@
 
 [Introduction](introduction.md)
 
-- [Notation](notation.md)
+- [Grammar notation](notation.md)
 
 - [Lexical structure](lexical-structure.md)
     - [Input format](input-format.md)

src/notation.md

@@ -1,5 +1,5 @@
 r[notation]
-# Notation
+# Grammar notation
 
 r[notation.grammar]
 ## Grammar