Update book/src/development/common_tools_writing_lints.md

Co-authored-by: Fridtjof Stoldt <xFrednet@gmail.com>

Author: nyurik

book/src/development/common_tools_writing_lints.md

@@ -16,7 +16,7 @@ Useful Rustc dev guide links:
 - [Type checking](https://rustc-dev-guide.rust-lang.org/type-checking.html)
 - [Ty module](https://rustc-dev-guide.rust-lang.org/ty.html)
 
-## Retrieving the type of expression
+## Retrieving the type of expressions
 
 Sometimes you may want to retrieve the type `Ty` of an expression `Expr`, for
 example to answer following questions: