Auto merge of #8547 - ThePuzzlemaker:patch-1, r=ehuss

Include `+` for crates.io feature requirements in the Cargo Book section on features

This change was introduced in commit 5f842f7 of rust-lang/crates.io but was not subsequently added to the Cargo Book.

Author: bors

src/doc/src/reference/features.md

@@ -87,7 +87,7 @@ optional dependencies. This allows packages to internally enable/disable
 features without requiring a new dependency.
 
 > **Note**: [crates.io] requires feature names to only contain ASCII letters,
-> digits, `_`, or `-`.
+> digits, `_`, `-`, or `+`.
 
 ### Usage in end products