Rollup merge of #67478 - brunobell:master, r=Centril

Fix src/libcore/str/mod.rs doc comments

Fix grammar in src/libcore/str/mod.rs doc comments. r? @steveklabnik

src/libcore/str/mod.rs

@@ -2325,7 +2325,7 @@ pub fn get_mut<I: SliceIndex<str>>(&mut self, i: I) -> Option<&mut I::Output> {
         i.get_mut(self)
     }
 
-    /// Returns a unchecked subslice of `str`.
+    /// Returns an unchecked subslice of `str`.
     ///
     /// This is the unchecked alternative to indexing the `str`.
     ///