Changed 0 into '0'

0 is not a production rule but a literal

src/libcollections/fmt.rs

@@ -327,7 +327,7 @@
 //! format := '{' [ argument ] [ ':' format_spec ] '}'
 //! argument := integer | identifier
 //!
-//! format_spec := [[fill]align][sign]['#'][0][width]['.' precision][type]
+//! format_spec := [[fill]align][sign]['#']['0'][width]['.' precision][type]
 //! fill := character
 //! align := '<' | '^' | '>'
 //! sign := '+' | '-'