doc: correct return type of Int32Value to int32_t

It currently reads uint32_t, which would be very strange, and
is also untrue (I checked napi.h) ;)

PR-URL: #459
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: NickNaso <nicoladelgobbo@gmail.com>

doc/number.md

@@ -43,7 +43,7 @@ Creates a new instance of a `Napi::Number` object.
 
 ### Int32Value
 
-Converts a `Napi::Number` value to a `uint32_t` primitive type.
+Converts a `Napi::Number` value to a `int32_t` primitive type.
 
 ```cpp
 Napi::Number::Int32Value() const;