style-guide: Make link text in SUMMARY.md match the headings in the linked pages

Author: joshtriplett

src/doc/style-guide/src/SUMMARY.md

@@ -2,10 +2,10 @@
 
 [Introduction](README.md)
 
-- [Module-level items](items.md)
+- [Items](items.md)
 - [Statements](statements.md)
 - [Expressions](expressions.md)
-- [Types](types.md)
-- [Non-formatting conventions](advice.md)
+- [Types and Bounds](types.md)
+- [Other style advice](advice.md)
 - [`Cargo.toml` conventions](cargo.md)
-- [Principles used for deciding these guidelines](principles.md)
+- [Guiding principles and rationale](principles.md)

src/doc/style-guide/src/cargo.md

@@ -1,4 +1,4 @@
-# Cargo.toml conventions
+# `Cargo.toml` conventions
 
 ## Formatting conventions
 

src/doc/style-guide/src/statements.md

@@ -1,3 +1,5 @@
+## Statements
+
 ### Let statements
 
 There should be spaces after the `:` and on both sides of the `=` (if they are