docs: remove "a" (#5499)

Was a duplicate article in the sentence. Already has "the"

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* Closes <https://github.com/emilk/egui/issues/THE_RELEVANT_ISSUE>
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crates/egui/src/util/id_type_map.rs

@@ -308,7 +308,7 @@ fn from_ron_str<T: serde::de::DeserializeOwned>(ron: &str) -> Option<T> {
 use crate::Id;
 
 // TODO(emilk): make IdTypeMap generic over the key (`Id`), and make a library of IdTypeMap.
-/// Stores values identified by an [`Id`] AND a the [`std::any::TypeId`] of the value.
+/// Stores values identified by an [`Id`] AND the [`std::any::TypeId`] of the value.
 ///
 /// In other words, it maps `(Id, TypeId)` to any value you want.
 ///