Rollup merge of rust-lang#37081 - p512:master, r=sfackler

Changed 0 into '0'

Right now `0` is an undefined production rule.
[Documentation following the grammar specification](https://doc.rust-lang.org/nightly/std/fmt/#sign0) strongly suggests `'0'` is meant as it is used as a character literal.

r? @steveklabnik

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 := '+' | '-'