Fix type expressed in last example of specs' Floating-point numbers (…

…fixes 370) (google#371)

According to the specs, aithmetic operations applied to a mixture of `float`
and `int` operands should result in a `float` value. However, the example
`3.0 // 2.0` had the value `1` commented beside it, which could imply
that its type is `int`. For clarity this mistake was fixed.

doc/spec.md

@@ -618,7 +618,7 @@ non-zero.
 3.0 / 2                                         # 1.5
 3 / 2.0                                         # 1.5
 float(3) / 2                                    # 1.5
-3.0 // 2.0                                      # 1
+3.0 // 2.0                                      # 1.0
 ```
 
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