Fix typos “an”→“a” and a few different ones that appeared in the same search

compiler/rustc_span/src/source_map.rs

@@ -100,7 +100,7 @@ pub trait FileLoader {
     /// Query the existence of a file.
     fn file_exists(&self, path: &Path) -> bool;
 
-    /// Read the contents of an UTF-8 file into memory.
+    /// Read the contents of a UTF-8 file into memory.
     fn read_file(&self, path: &Path) -> io::Result<String>;
 }