Include + for crates.io feature requirements

This change was introduced in commit 5f842f7 of rust-lang/crates.io.

Author: ThePuzzlemaker

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