Corrected EBNF grammar for from_str

Previously, the `Number` part of the EBNF grammar had an option for `'.' Digit*`, which would include the string "." (a single decimal point). This is not valid, and does not return an Ok as stated. The corrected version removes this, and still allows for the `'.' Digit+` case with the already existing `Digit* '.' Digit+` case.

library/core/src/num/dec2flt/mod.rs

@@ -126,7 +126,6 @@ macro_rules! from_str_float_impl {
             /// ```txt
             /// Float  ::= Sign? ( 'inf' | 'infinity' | 'nan' | Number )
             /// Number ::= ( Digit+ |
-            ///              '.' Digit* |
             ///              Digit+ '.' Digit* |
             ///              Digit* '.' Digit+ ) Exp?
             /// Exp    ::= 'e' Sign? Digit+