Correct double literal example (#414)

`7.` is not in fact a valid double literal per the grammar or the `cel-go` implementation. But `.700e1` is, and shows off some other aspects of flexibility in the grammar.

doc/langdef.md

@@ -247,7 +247,7 @@ floating-point. We only support positive, decimal integer literals; negative
 integers are produced by the unary negation operator. Note that the integer 7 as
 an `int` is a different value than 7 as a `uint`, which would be written `7u`.
 Double-precision floating-point is also supported, and the integer 7 would be
-written `7.`, `7.0`, `7e0`, or any equivalent representation using a decimal
+written `7.0`, `7e0`, `.700e1`, or any equivalent representation using a decimal
 point or exponent.
 
 Note that currently there are no automatic arithmetic conversions for the