fix typo in doc comment

Author: lqd

compiler/rustc_middle/src/mir/pretty.rs

@@ -1952,7 +1952,7 @@ pub(crate) fn pretty_print_const_value<'tcx>(
 ///////////////////////////////////////////////////////////////////////////
 // Miscellaneous
 
-/// Calc converted u64 decimal into hex and return it's length in chars
+/// Calc converted u64 decimal into hex and return its length in chars.
 ///
 /// ```ignore (cannot-test-private-function)
 /// assert_eq!(1, hex_number_length(0));