Fix clippy warning about misleading comment

Author: ogoffart

cpp_common/src/lib.rs

@@ -225,7 +225,7 @@ pub enum Macro {
 }
 
 impl Parse for Macro {
-    ///! Parse the inside of a `cpp!` macro (a literal or a closure)
+    /// Parse the inside of a `cpp!` macro (a literal or a closure)
     fn parse(input: ParseStream) -> Result<Self> {
         if input.peek(syn::token::Brace) {
             let content;