Reword "pinned box" to "pinning box"

The official documentation of the `core` crate uses the term "pinning
box" instead of "pinned box". This commit updates the documentation to
use the correct term.

yash-syntax/src/input.rs

@@ -77,7 +77,7 @@ pub trait Input {
     /// Errors returned from this function are considered unrecoverable. Once an error is returned,
     /// this function should not be called any more.
     ///
-    /// For object safety, this async method is declared to return the future in a pinned box.
+    /// For object safety, this async method is declared to return the future in a pinning box.
     async fn next_line(&mut self, context: &Context) -> Result;
 }
 

yash-syntax/src/parser/lex/core.rs

@@ -759,7 +759,7 @@ impl<'a> Lexer<'a> {
         Ok(self.core.source_string(begin..end))
     }
 
-    /// Like [`Lexer::inner_program`], but returns the future in a pinned box.
+    /// Like [`Lexer::inner_program`], but returns the future in a pinning box.
     pub fn inner_program_boxed(&mut self) -> Pin<Box<dyn Future<Output = Result<String>> + '_>> {
         Box::pin(self.inner_program())
     }

yash-syntax/src/parser/list.rs

@@ -191,7 +191,7 @@ impl Parser<'_, '_> {
         }
     }
 
-    /// Like [`maybe_compound_list`](Self::maybe_compound_list), but returns the future in a pinned box.
+    /// Like [`maybe_compound_list`](Self::maybe_compound_list), but returns the future in a pinning box.
     pub fn maybe_compound_list_boxed(
         &mut self,
     ) -> Pin<Box<dyn Future<Output = Result<List>> + '_>> {