Fix capitalisation in Path#file_name's docs

Author: dwijnand

src/libstd/path.rs

@@ -1825,7 +1825,7 @@ impl Path {
     /// If the path is a normal file, this is the file name. If it's the path of a directory, this
     /// is the directory name.
     ///
-    /// Returns [`None`] If the path terminates in `..`.
+    /// Returns [`None`] if the path terminates in `..`.
     ///
     /// [`None`]: ../../std/option/enum.Option.html#variant.None
     ///